MANUAL OF ZOOLOGY. 



developed on one side of the stomach, rapidly increases in 

 size, envelops the stomach, which it appropriates, and is ulti- 

 mately converted into the adult Echinoderm ; the remainder of 

 the larva being absorbed or cast off as useless. 



The essential peculiarity of the development of the typical 

 Echinoderms, as above summarised, is that the larva possesses 



provisional organs, which 

 may be ultimately absorbed 

 or thrown off, but which are 

 not converted into the cor- 

 responding structures of the 

 adult. Thus the larva of an 

 Eehinoid (fig. 92) possesses 

 a mouth and alimentary ca- 

 nal, which are not converted 

 into, 'and in no way corre- 

 spond with, the mouth and 

 alimentary canal of the adult. 

 The larva, or " pseudem- 

 bryo," as it is termed by Sir 

 Wyville Thomson, leads a 

 perfectly independent exist- 

 ence, and the true Echino- 

 derm is produced from it by 



Fig. 92 .-Larva of Echinus (after J. Muller). a prOCCSS of internal budding 



A A, Front arms with their internal skele- or rearrangement, 

 ton; F F, Arms of the mouth-process; B, /-,- , r ,, rpi \ 



Posterior side-arm; a Mouth; a' CEsopha- Sir WyVllle ThomSOll has, 



gus; 3 stomach; v intestine \d ciliated further, shown that there are 



bands ; ff Ciliated epaulets ; c Disc of the 



future Echinus. various cases amongst the 



Echinoidea, Asteroidea, Ophi- 



uroidea, and Holothuroidea, in which the young are developed 

 directly from the egg, without the intervention of a locomotive 

 pseudembryo. In these cases, the eggs are hatched, and the 

 young are brought up, " within or upon the body of the parent, 

 and are retained in a kind of commensal connection with her 

 until they are sufficiently grown to fend for themselves." There 

 is no sort of organic connection in these cases between the young 

 and the parent ; but the young are often brought up in a 

 special receptacle upon the exterior of the mother, to which 

 the appropriate name of the " marsupium " has been given. 

 This viviparous mode of reproduction seems to obtain specially 

 among the Echinoderms of the cold northern and southern 

 seas. 



The Echinodermata are divided into seven orders viz., the 

 Crinoidea, Cystoidea, Blastoidea, Ophiuroidea, Asteroidea, Echi- 



