274 



COMPA RA TI VE A NA TO MY 



CHAP. 



found. The mid-gut is the only part of the intestine which is developed. Between 

 the intestine and the outer integument of the larva are found the paired secondary 

 body cavity, mesenteries, and muscles. The larva attaches itself by the end of the 

 posterior segment which grows out into a stalk. The two folds of the middle 

 segment bend forward like valves and form the mantle, the reduced anterior segment 

 coming to lie in the mantle cavity. The bundles of sette are thrown off. The 

 stomodseum is formed by an imagination of the body wall of the anterior segment, 

 whose base breaks through into the anterior end of the mid-gut. It lies a little 

 below the eyes which afterwards degenerate. The rudiments of the nervous system 

 and of the nephridia have not been observed. In Terebratulina tentacles develop 

 as buds on the circular edge of a disc which projects from the dorsal mantle fold. 

 The tentacles increase in number and are grouped in the shape of a horse-shoe. The 

 tentacular disc is then anteriorly prolonged into two processes, the arms, on which 

 the double row of tentacles become the arm cirri. We here recognise great agree- 

 ment with the Bryozoa and Phoronis. Assuming that the point where the eyes lie 

 on the umbrella-shaped anterior segment in the, Braeliiopod larva corresponds with 

 the neural plate of Actinotrocha, the agreement between their courses of development 

 is considerable. The posterior segment of the Brachiopod larva perhaps answers to 

 the evaginating trunk part (stalk) in Actinotrocha. The oral nerve rings in the 

 Brachiopoda and in Phoronis must be homologous. 



A free-swimming larva which occurs among the Nemertina is called Pilidium (Fig. 



185). It is helmet-shaped. We can dis- 

 tinguish in it a convex aboral region and 

 a somewhat concave oral region. The 

 ciliated ring is found on the boundary 

 between these two, which corresponds 

 with the edge of the helmet. Along the 

 base of the ciliated ring runs a nerve. 

 The ciliated ring, which answers to the 

 Trochophoran preoral ciliated ring, is 

 produced to the right and left into two 

 pendent lobes, which are comparable 

 with the ear-flaps of many helmets. At 

 the highest point of the aboral region 

 is a depression ; in it the ectoderm is 

 thickened, and carries a ciliated tuft. 

 FIG. i85.-Pilidium- Larva ot a Nemertian This answers in position to the neural 

 from the side (after Salensky). sp, Neural thick- plate of the Trochophora. The mouth 

 ening with ciliated tuft ; wk, ciliated ring ; rn, lies ill the middle of the oral region, 

 nerve of the same ; m, muscles ; st, stomodaum ; j t leadg into the S t moda?imi, and this 



md, mid-gut ; es, ectoderm sacs. . , -, ., . , -u -u i 



into a sac-like mid-gut which lies ex- 



centrically (behind). A proctodseum is wanting, and does not attain development 

 during larval life. The space between the intestine and the ectoderm is a 

 spacious segmentation cavity or primary body cavity. In it lie muscle fibres, and 

 generally branched star-like mesoderm cells. The ectoderm of the oral region is 

 invaginated into the primary body cavity at 4 points, forming 2 pairs of sacs. One 

 pair of these sacs lies in front of and the other behind the stomodanim. The 

 further development of these sacs is as follows (Fig. 186, A, B, C, D) : They sever 

 themselves from the ectoderm of the larva. They then become connected in 

 pairs, then the anterior fused pair unites with the posterior pair, so that now, 

 on the oral side of the Pilidium, in its body cavity, a hollow plate arises with 

 inner and outer walls. The inner wall grows round the enteric canal on all sides, 



