MUSEUM OF COMPARATIVE ZOOLOGY. 115 



the blastopore, and the gastrula cavity or future intestine, as be confesses 

 practically in a later paper.* 



The following quotation from Apostolides leads me to suspect that this 

 author did not clearly distinguish the mesoblastic cells in his young Am- 

 phiura with a single cavity (intestine) and single opening (anus). Possibly 

 he confounded them with the hypoblast. He says :t "Dans I'ectoderme 

 on commence a apercevoir quelques points oranges premiers indices 

 de calcification." If these calcifications are really formed in the ecto- 

 derm or epiblast, it is exceptional among larval Echinoderms, where they 

 are regularly formed in the mesoblastic cells. Metschnikoff;}: rightly in- 

 terprets the orange cells of the young Amphiura as mesoblastic or "cutis" 

 cells. There is nothing in Apostolides' account either of Ophiothrix or 

 of Amphiura to show that he recognized these cells which play such an 

 important part in the growth of certain parts of the larval Echinoderm. 



A study of Metschnikoff" 's Fig. 6. PI. III.^ is an instructive one. In 

 this figure the epiblast (ep) is well formed, and the opening (an opening 

 not figured, § but described in the text) on the lower pole is identified with 

 the mouth opening of the older larvae, which assume a bilaterally sym- 

 metrical contour. The young Amphiura is already bilaterally symmetrical, 

 for on each side of the "opening" can be seen the beginning of the 

 vaso-peritoneal vesicles, or water-tubes (v). At the pole opposite the 

 supposed mouth there are trifid bodies (cc), identified as the provisional 

 spines of the pluteus. 



The homology of the parts of the larva mentioned has given me much 

 trouble ; for if we regard the SQ-called mouth as the original opening or 

 future anus, the positions of the water-tubes and spines as compared with 

 the same stage in other Echinoderms are wrong. If the opening in ques- 

 tion (mv) is a mouth or second infolding of the epiblast, where are the intes- 

 tine and the anus 1 In his admission * that Apostolides is right in his 

 interpretation of the homology of intestine and anus Metschnikoff" must 

 have abandoned the idea that the interpretation of this figure is a correct 

 one so far as the opening mv is concerned. I have found a larva similai- 

 to the figure quoted (Fig. 6), vnth the two transparent bodies (v) which 



* Zeit. f. Wiss. Zool, XXXVII. p. 307. 



+ Op. ciL, p. 210. 



X Studieii iiber die Entwickelung der Echinodermen und Nemertinen, Mirfi. 

 de I' Acad. Imp. dcs Sci. de St. Petersb., VIP ser. XIV. 8. 



§ There is a mistake in letteriug and in de3cription. On p. 15 mv is spoken of 

 as mouth, mv in the explanation of Figs. 3, 4, 5, is the vitelline membrane ("Dot- 

 terhaut"). 



