LANt'KLET-KISHKS. 



lL>l'.l 



i!\w liiTc anil their exiihiiiMtiuiis. In tlic last stages 

 tli(^ external t'urni (if the liddy is aliiuist complete. But 

 it still remains tor the month to assume its ventral 

 position and its wreath ot tentacles instead of the long 

 vibratile hairs (not included in our figures) which are 

 set during the larval ])eriod on a vei'inicifonn thicken- 

 ing just in front of the month-ajiei-lure. 



Ilfanclihistoniii hesides teaches us the apjiearance 

 presented 1)\' the locomotive oi'gans (jf the vei'tehrates 

 in th{?ir simplest form. riie locomotive organ proper 

 is the hin<l part of the liody, edgc^l ahoNc and lielow 

 by a kind of caudal lin or rather a compound of i-udi- 

 ments of the posterior part (jf a dorsal fin, a caudal 

 fin, and an anal fin. iiefore the anus this fin-gi-owth 



■::iv.^\.fi<^- 



end vel cil 



Fit;. 3"!'- A larva 5 iiiiii. long, taken togetlier wifli tlic original of tig. 37^. From a pri'paration hrietly tn-atrct with a weak snlntion of 

 osniinni bielironiftte by Prof. G. Retzius and preserved in glycerine. j4, from the right; B, from the left. Here the above-mentioned fold (et)) 

 originating in the inferior wall of the intestinal canal to foini the epibranchial (hyperbranehial) groove has divided into two parts, the one 

 (ep) following the tops of the branchial arches, the other (ep") taking its course up towards the roof of the abdominal cavity. In B the 

 larval mouth shows the coniniencenient of its alteration and translation from the left side of the body to the ventral margin. A dermal fold 

 (or) has laid itself over the upper part thereof; and the superior margin shows signs of dissolution, atrp, atriopore; metd, edge of rio'ht 

 wefs, of left metapleure; n', first pair of nerves, with a few ganglion cells (ef. fig. 370, e) visible; other letters as in the preceding figure. 



Fig. 380. Diagranmiatical transverse section, A, through a larva of Branchiostoma lanceolatum, with 11 or 12 primary gill-slits, and B, 



through a full-grown specimen of the same species. Enlarged. After R.4Y Lankester and Willey. 

 dm, myoecel of the dorsal fin (that part of the body-cavity (coilotn) which belongs to the dorsal fin-lobe); fr, series of fin- rays; m, great 

 lateral nniscles; mij, muscle ciBlom, rayoco/l (that part of the body-cavity which belongs to the lateral muscles); m\ spinal cord, with its 

 ' cntral canal; nch, notochord; d. a., dorsal aorta; v. a., ventral vessel; Int, intestine; sp, splanchnocoel (intestinal part of the abdominal ca- 

 \ity), which (in B) is divided by the ingrowth of the atrial chamber into an inner {sp') and an outer {sp") cavity; sp'" (in B), perigonadial 

 dilatation of the splanchnoctcl ; d. I. m., double-layered membrane separating the myoc<el from the splanclinocffl; at, atrium; met, metapleure, one on 

 each side of the body, its cavity, according to Ray Lankester, not in open communication with the ctelom; s. a. r., floor of the atrial chamber. 



