366 



associated "the larva of Loven" (which Mr. Agassiz has traced 

 without any doubt to the Nemertean worm) with that of Polynoe, 

 a representative of the highest family of Chaetopod worms. In 

 the first stage (Fig. 171, a, anus: c, intestine; m, mouth; o, 

 oesophagus ; a, stomach ; e, eye-speck ; v, ciliated ring) the larva 

 is not ringed ; this figure may be compared with figure 96 on p. 

 231 to show how much alike the worm and Echinoderm larvae 

 appear. The new rings are formed between the anal rings and the 

 older anterior rings, as in annelid larvae, and in fact in the em- 

 bryos of the Insects and Crustacea. Figs. 172 and 173 represent 

 the ringed larva. "A number of rings make their appearance at 

 once, and are the more distinct the nearer they are placed to the 

 mouth." The worm now greatly elongates, more segments are 

 added and it appears as in Figs. 174 and 175, with the ciliated 

 crown, the small short tentacles and eyes. The worm now swims 

 about slowly and creeps over the bottom, and is nearly a quarter 

 of an inch long. It will be observed that the larva differs from 

 those of other Annelides, as Mr. Agassiz states, in the absence of 

 "feet, bristles or appendages of any sort, except the two tentacles 

 of the head ; and, were it not for these, it would seem as if the 

 young worm were the larva of some Nemertes-like animal." Fig. 

 176 represents the worm over four months after the stage rep- 

 resented by Fig. 175, the articulations have disappeared an* a 

 month later the head is separated from the body by a neck, the 

 tentacles disappear, the body is flattened, and the Nemertes 

 (Polia) form is attained. 



It is thus interesting to know that the young Echinoderm (Fig. 

 96), the young mollusk (Fig. 140 B) and the young Nemertean 

 worm pass through a similar free swimming Cephalula stage. ^ e 

 shall see farther on that the young Balanoglossus and the true 



should be borne in mind that different species pass throng" 

 ferent cycles of growth, some exhibiting no metamorphosis, 

 stages being more or less condensed in the embryo state. 

 1- Egg. 



2. Morula. 



3. Planula (or Gastrula?) hatching as a 



4. Ciliated Infusorian-like larva, or a 



5. Pilidium or a Cephalula. 



