MUSEUM OF COMPARATIVE ZOOLOGY. fd 
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 highly modified so as to lose all semblance to these or- 
gans in other Echinoderm larve. ‘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 Metschnikoff. 
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 larvee (Figs. 
4-11) with others which have been figured by other observers. In the 
papers by Apostolides and Metschnikoff the position of figures of the same 
larva is different, 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 Wassergefiisssystem 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 “ Nabelschnur”’ 
(uw). It is at first, in young larval stages, broad and thick, and later 
becomes reduced in size to a simple string-lke structure. Its existence 
prior to the pentagon-shaped larva is not indicated by Apostolides and 
Metschnikoff, although represented by Max Schultze. Metschnikoff+ 
says: “Erst auf solchen Stadien (Fig. 17) kénnte ich deutlich den 
von Krohn und Max Schultze bereits gesehenen Strang beobachten 
welcher sich dem das provisorische Skelet tragenden Korpertheile des 
Embryo inserirt.” In the early conditions (Figs. 5, 8, 9) of the bisymmet- 
¥ Op. cit., p. 16. T Op. cit., p. 18. 
