of the Echinoderms. 9 



structure ultimately throws off more or less of the larva in which 

 it was developed, or it unites with the larva to form the adult 

 animal, no part being thrown off. 



The former is the case in the Ophiuridye, Echinidae and Astc- 

 ridse, for the most part — the latter in the Holothuriadse. 



The latter process, as the simpler, shall be described first. 



A portion of the dorsal integument of the larva becomes as 

 it were thrust inwards (fig. 10.) towards one or other side 

 of the stomach, as a tube terminated by an enlarged globular 

 extremity, whose cavity communicates with the exterior and is 

 ciliated internally. 



The vesicle which terminates this " internal bud " now sends 

 forth processes so as to form a sort of " rosette," which lies close 

 to and above the stomach. 



The " rosette " becomes a circular canal (the circular canal of 

 the water- vascular system), from which caeca are given off ante- 

 riorly to form the tentacles, posteriorly to the parietes, in which 

 they become the water-canals. 



The former mouth of the larva is obliterated, and a new one 

 is formed in the centre of the circular canal and its tentacular 

 appendages. This is the permanent mouth of the Holothuria, 

 which is therefore a new structure formed upon the dorsum of the 

 larva. 



In the meanwhile, vesicles, the Vesiculse Polianse, are deve- 

 loped from the circular canal, and a deposit of calcareous matter 

 takes place round a portion of the tubular canal, from whose 

 spherical extremity the water- vascular system has been formed. 

 That portion of the tubular canal which lies between the dorsal 

 parietes and the calcareous deposition dies away, and the re- 

 mainder hangs freely from the circular canal of the water-vas- 

 cular system as the u sand-canal." 



The process in the Echinidse, Asteridae, and Ophiuridse is es- 

 sentially the same ; only, as in these the old body is to be more or 

 less completely discarded, the development of the water -vascular 

 system is attended, pari passu, by that of a mass of cells from 

 which the new body is to be formed. 



We cannot do better than adduce in illustration Prof. Muller's 

 description of the development of the Echinoderm in the Asterid- 

 larva Bipinnaria (Fortsetzung der Untersuchungen iiber die Me- 

 tamorphose d. Echinodermen, Mull. Archiv, 1850). 



In larvae which are not 0*15 of a line in length, the dorsal pore 

 and the tube which proceeds from it are perceptible. It passes 

 into a longish sac, in which, as in the tube, there is a ciliary 

 motion. The sac lies behind, at the side of the oesophagus 

 (Diag. IX.). 



Soon offer the appearance of these parts, a hyaline mass, in which 



