OF ZOOPHYTA. 291 



the adult. The embryos which I am about to describe were 

 hatched in March, from the eggs of Holothuria tremula, a 

 Trepang which lives in the seas about the coasts of Norway. 

 Like the scaly Trepang, Psolus phantapus, which I alluded to in 

 a previous lecture, (p. 192, fig. 117,) it has rows of sucker-like feet, 

 on the lower side of the body, and great branching feelers pro- 

 jecting from the head ; but the upper side of the body is soft, and 

 covered with numerous protuberances. Interiorly the organiza- 

 tion of this Holothurian is, in a general way, like that of the 

 flesh-colored, footless Trepang, Caudina, (p. 187, fig. 114,) of our 

 coast ; and the latter, therefore, may serve as a means of com- 

 parison in tracing the relations of the developing organs of the 

 young of the former. 



The mode of formation of the parts in the earliest stage of 

 growth, after the egg has passed through the phase of self-divis- 

 ion, is very much like that which occurs in the Sea-anemone ; i. e., 

 the globular so-called mulberry-mass becomes indented at one end 

 and finally hollowed within, so as to form a sort of vase with a 

 narrow mouth. In the course of a week the body assumes an 

 oval form, and several of the organs about the head begin to 

 take on distinct shapes and outlines. In this figure, (fig. 191,) 

 which represents the upper side of an embryo at the 

 age just mentioned, the mouth (m) is in profile at 

 the extreme end of the head, and just behind it three 

 of the five incipient feelers are visible, standing in 

 such a position that one is in the mid-line of the back, 

 and the others are respectively right and left of it. As 

 we shall see presently in an older phase, the remaining two of 

 the five have the same relation to the inferior mid-line as the 

 superior right and left ones have to the upper median line. A 

 short distance behind the upper odd tentacle, and in the same 

 line, there is a little pit (me) which forms the end of a canal 

 within, that extends from it to a hollow ring or channel about 



Fig. 191. Holothuria tremula. Gunn. Magnified. An embryo soon after 

 birth, seen from the back, m, the mouth ; t, the incipient feelers ; me, the first 

 trace of the madreporic body. 1'rum Koren and Danielssen. 



me 



