270 



COMPARATIVE ANATOMY 



CHAP. 



the mesoblast which lie near the posterior end ; these lie close to the ectoderm 

 ventrally on each side, and develop anteriorly by constricting off smaller cells and by 

 the continuous division of the same ; they are the two xnesoderm streaks. The larva 

 has now reached what is known as the Trochophora stage. It swims about freely 

 by means of its ciliated rings. The hindermost smaller portion of the body with 

 the intestine, proctodseum, and the posterior part of the mesoderm streaks, represents 

 the rudiment of the afterwards segmented trunk plus the anal segment ; the whole 

 of the larger anterior portion contains the rudiment of the head or oral segment. 



Larvre of the Trochophora type occur not only in worms, but are also common 

 among Molluscs. 



In addition to the Trochophora the following are some of the most important 

 kinds of worm larvse. 



The free-swimming larva of the Sipunculidce (Fig. 179) is already much further 

 developed when hatched than the Polychcetan Trochophora, with which, however, 

 it has much general similarity. In the Sipunculus larva the characteristic 

 preoral ciliated ring of the Trochopliora larva is wanting, but such a ring, weakly 



md 



sp 



FIG. 179. Larva of Sipunculus, after Hatschek. o, Mouth ; sp, neural-plate ; sto, stomodseum 

 wk, post-oral ciliated ring; rr, retractors of the anterior body (proboscis); md, mid-gut; n 

 nephridium ; an, anus. 



developed, is said to occur in the Phascolosoma larva. A postoral ciliated ring, on 

 the other hand, is strongly developed. The intestine consists, as in the Trochophora 

 larva, of stomodseum, mid-gut, and proctodaeum. The latter is moved somewhat 

 from the posterior end on to the back, and the postanal part of the body growing 

 faster than the preanal part, the anterior position of the anus which is characteristic 

 of the adult animal comes about. The neural plate with 2, and later 4, larval 

 eye-spots is found in the same place as in the Polychcetan Trochophora. A larval 

 head kidney is not formed. The mesoderm, on the contrary, is far more developed 

 than in the Polychcetan Trochophora. The retractors of the anterior body (proboscis), 

 and the two trunk nephridia, have already begun to form. There is a spacious 

 body cavity, but this does not correspond with the primary body cavity of the 

 Trochophora, but rather with the secondary body cavity of the Annulata which 

 appears within the mesodermal streaks, so that under the integument and around 

 the intestine there is already a layer of mesoderm cells answering to the parietal 

 and visceral layers of the mespderm of the Annulata. While, however, in the 

 Annulata the mesodermal streaks become segmented and the body cavity is thereby 

 divided into consecutive pairs of chambers, in the Sipunculidce there is neither 

 segmentation of the mesoderm nor of the body cavity. 



