I have an indistinct recollection of catching another larger No. This space opens behind, directly into the blastocoelcavity. The body is divided into three parts: 1 proboscis protosome, 2 collar mesosome , and 3 trunk metasome. The females extrude egg masses onto the surface of the substrate from within their burrows 2,000 to 3,000 eggs at a time. T h e digestive tract is indistinctly seen through the body walls as a simple tube. Each is filled with many and exceedingly small nuclei. U-shaped pre oral ciliary organ tests the quality of food and water entering the mouth.
The secondary pharyngeal bars have a coelomic canal derived from the trunk coelom. The nervous layer is separated internally by a well-developed basement membrane. There was no evidence that the young worms absorbed any of the carmine, and the experiment gave entirely negative results. The details of the buccal diverticulum have already been discussed under the heading—Supporting Structures. We come next to a stage where the larva has rcachcd its minimum size. The name Balan oglossus had thus become firmly embedded in zoological literature when it was stated Ptychodera must be used instead.
Acorn worms may be deposit feeders the majority of the burrowing species , suspension feeders nonburrowing species , or both. They feed on detritus to extract the organic material. The main differences are: 1. Behind this the digestive tract is a small tube, opening at the posterior end of the body. The a n t e r zvalZ o the oeso~hngzrsis b z f far,? Cilia push water through the slits, maintaining a constant flow. A Para pharyngeal ridge on each side divides the cavity of the pharynx into two incomplete halves—a dorsal respiratory half pierced by gill-slits and a ventral half that helps in food concentration see Fig.
The middle arm of an M-shaped rod is bifurcated at the free end and lies in a gill-septum, while its outer arms lie in adjacent tongue bars. The food particles are transferred to the ventral, i. The body is bilaterally symmetrical and divided into three regions, viz. From the blood-vessel the blood is continuous with the irregular spaces between the inturned cells of the probosciscavity. Its tongue-bar is forming by inturning of the dorsal wall. T h e tip of the proboscis was thrust obliquely or even vcrtically downwards between the coarse grains of sand. I'd follow Ptychoderidae for parentheses, but the species lists need to be reconciled; the list at the family article has species not found on the list at the genus article and vice versa.
This is the collar nerve-tube. The largest acorn worm Balanoglossus gigas reaches is more than 2. The coelom of the trunk is divided into two lateral closed cavities by vertical partition. Its food comprises of microscopic organisms and organic particles present in water and the bottom sand in which it makes its burrows. The mucus is secreted by the gland cells of the proboscis epithelium.
However, the data can also be explained by a common ancestor with diffuse dorsoventral organization, followed by oppositely directed condensation of the nerve cord and relocation of the heart in the two lines. The gill-slits have little to do in trapping of the food particles. There are reasons for supposing that the truncal coelom was at one time provided with pore-canals, but supposed vestiges of these structures have only been described for one genus, Spengelia,in which they lie near the anterior end of the truncal coelom. The larva of Hemichordata and Annelida also differ in the following ways: i Nephridia are absent in tornaria larva. The inner side of the body wall is lined with parietal coelomic epithelium. W e have, fortunately, a well-defined and important landmark by which to gauge the growth of this region. This fact indicates that the digestive tract is to undergo farther changes before it gets into its final position, T h e anterior body-cavity has enlarged, and may be dimly seen through the walls of t h e body.
Along the course of the ciliated band No. To the right of the hydropore lies a pulsating heart vesicle which develops in the later stages of tornaria larva. Delle Chiaje first recorded the animal from the Naples region in 1829 and gave the name Balanoglossus clavigerus. Division of body Externally the body is divisible into three regions : The proboscis or protosome, collar or mesosome and trunk or metasome. T h e second and third pairs of body cavities were not observed in the living animal, but sections show their presence, and show them to have enlarged between this and the last stage. In the present discussion, the Hemichordata has been given the status of a phylum. However modern science has proven that none of them posses a post anal tail or a notochord and they have been allocated to a phylum of their own.
The meaning of this singular contrast between the two animals may be that we have here an instance of an interesting gradation in evolution. This mucus string is directed towards the pre-oral ciliary organ at the base of the proboscis by the cilia covering the proboscis. In the neck of the proboscis the fibrous layer is greatly thickened. The Tornaria larva offers a certain similarity to larvae of Echinoderms sea-urchins, star-fishes, and sea-cucumbers , and when first discovered was so described. The notochord is large, and fills up much of the interior of the narrow neck. The reproductive organs are mere groups of cells situated in the sides of the trunk—in the flaps when these are present.
Three pairs of gill-pouches are seen projecting from the dorso-lateral walls. Chordata is composed of three subphyla. That they are modified mesenchyme cells, I think, there can be no doubt ; but they are immensely larger than the mesenchyme cells, as may be seen in the figure. The nervous layer becomes thickened along definite strands to form two main nerve cords, one mid-dorsal and the other mid-ventral, which run along the entire length of the trunk. A narrow crevice is left in the middle line above the fused ectoderm, and was seen at the surface as a longitudinal furrow. I t is clivided into four subregions : the ventral-lateral area, the dorsal-lateral area, the middle-lateral area, and the posterior-ventral area. The space contains a coagulated material resembling blood, and the space seems to be a blood-vessel.