MUSEUM OF COMPARATIVE ZOOLOGY, 117 



larva an anal opening into the intestine, that opening would be function- 

 ally useless, for it is closed by the sac in which the embryo hangs. Be- 

 fore the young sever their attachment to the parent, anus and intestine 

 are atrophied or higlily modified so as to lose all semblance to these or- 

 gans in other Echinoderm larvae. The mouth also in very early stages, 

 as can be observed by an examination of Figs. 5, 7, is closed by the sac in 

 which the larva is suspended. It would not seem strange if, in the pos- 

 sibly abbreviated development which is found in Amphiura, a true anal 

 opening never forms, and that the primitive gastrula cavity is formed not 

 by invagination but by delamination. The intestine and anus in stages 

 corresponding to Fig. 4 are not figured by Metsclmikoff. 



For a considerable time, and almost through the whole course of the 

 development of the bilateral embryo, a conspicuous cluster of orange pig- 

 ment is found at the anal pole of the larva near the intestine. 



There are some difficulties in a comparison of the bilateral larvoe (Figs. 

 4-11) with others which have been figured by other observers. In the 

 papers by Apostolides and Metsclmikoff" the position of figures of the same 

 larva is diff"erent, and Metschnikoff" does not follow in his figures the ori- 

 entation given in the text. He says:* "Wenn man sich den Embryo 

 mit dem Esophagus nach oben, den Magen nach unten liegend denkt, so 

 wird sich die Wassergefusssystem Anlage auf der linken Seite befinden." 

 This is a position exactly opposite that in which he has placed his fig- 

 ures. As the mouth on a median line is a convenient point for reference, 

 its position ought to be mentioned when speaking of the position of the 

 larva. When the mouth is turned to the observer and the anal end of 

 the larva is above, the right water-tube (right-hand side of larva) is that 

 which divides into five divisions. When the mouth is turned from the 

 observer and the anal pole placed above, the left water-tube is that which 

 divides to form the water-vascular system. 



Umbilicus. — The connection of the bilateral larva with the ovary is by 

 means of a structure which may be called the umbilicus, or "ISTabelschnur" 

 («). It is at first, in young larval stages, broad and thick, and later 

 becomes reduced in size to a simple string-like structure. Its existence 

 prior to the pentagon-shaped larva is not indicated by Apostolides and 

 Metschnikoff", although represented by Max Schultze. Metschnikoff" t 

 says: "Erst auf solchen Stadien (Fig. 17) konnte ich deutlich den 

 von Krohn und Max Schultze bereits gesehenen Strang beobachten 

 welcher sich dem das provisorische Skelet tragenden Kcirpertheile des 

 Embryo inserirt." In the early conditions (Figs. 5, 8, 9) of the bisynimet- 

 * Op. cit., p. 16. t Op. cit., p. 18. 



