( 'OMI'A RA Tl \ *E A X. 1 TOM Y 



CHAP. 



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



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

 !>ody 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 seta' are thrown oil'. The 

 stomodieum is formed by an imagination of the bodv 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 nepliridia have not been observed. In Tercbratnlina 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. AYe here recognise great agree- 

 ment with the Jjt-yo'.na and PJiOi'onis. Assuming that the point Avhere the eyes lie 

 on the umbrella-shaped anterior segment in the .Jirucltinjiod larva corresponds with 

 the neural plate of ^-Iffiimf rue/at, the agreement between their courses of development 

 is considerable. The posterior segment oi t\\& Bmcltiopod larva, perhaps answers to 

 the evaginating trunk part (stalk) in A<:tuiotrudia. The oral nerve lings in the 

 I!r(i<-lt!i>jivd<i and in P/toro/d* must lie homologous. 



A five-swimming larva which occurs among \\\G 2?e inert ina is called Pilidium (Fig. 



1 sr. ). It is helmet-shaped. AVe 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 

 Trochophvran preoral ciliated ring, is 

 produced to the right and left into two 

 pendent lobes, which are comparable 

 with the ear-Haps of manv helmets. At 

 the highest point of the aboral region 

 is a depression ; in it the ectoderm is 

 thickened, and- carries a ciliated tuft. 

 I'm. is:,. -Pilidium- Larva ol a Nemertian T!lis answers in position to the neural 

 hum the side (after Salensky). >'/s Neural thick- plate of the T I'ocln^iJnu'd. The mouth 

 i-iiiiiH with ciliated tuft; icfr, ciliated riii- ; rn, lies in the middle of the oral region. 

 n.Tvcof the same; m , muscles; st, stomod*um ; lf , ( , 1(]s j ]lto t]u . st omodieum 5 and this 

 u/'l, niid-L'ut ; c.s- ectoderm sacs. . ,., .-, i i v 



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



eentricallv (behind). A proctod.enm is wanting, and does not attain development 



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

 >pacioiis segmentation cavity or primary body cavitv. In it lie muscle fibres, and 

 Lfeiiera'] v branched star-like mesoderm cells. The ectoderm of the oral region is 

 invaginated into the ]irimary body cavity at I ]ioints, forming 'J ] tail's of sacs. One 

 pair of these sacs lies in front of and the other behind the stomod;eum. The 

 further development of these sacs is as follows (Fig. 18(5, A, ]>, C, ]>}: 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 Pil'l iti.m, in its body cavity, a hollow plate arises with 

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



