ANNTJLOIDA I ECHINODEKMATA. 123 



anal aperture ; the whole being ' disposed in a longitudinal 

 and vertical plane, dividing the larval body into two symme- 

 trical halves.' Besides the digestive canal, no other organs 

 have hitherto been discovered in these larvae. In the further 

 process of development, 'an involution of the integument 

 takes place upon one side of the dorsal region of the body, so 

 as to give rise to a csecal tube, which gradually elongates 

 inwards, and eventually reaches a mass of formative matter, 

 or blastema, aggregated upon one side of the stomach. 

 Within this, the end of the tube becomes converted into a 

 circular vessel, from which trunks pass off, radially, through 

 the enlarging blastema. The latter, gradually expanding, 

 gives rise in the Echinidea, the Asteridea, the OpJmiridea, and 

 the Crinoidea to the body- wall of the adult ; the larval body 

 and skeleton (when the latter exists), with more or less of the 

 primitive intestine, being either cast off as a whole, or dis- 

 appearing, or becoming incorporated with the secondary 

 development, while a new mouth is developed in the centre of 

 the ring formed by the circular vessel. The vessels which 

 radiate from the latter give off diver tic ula to communicate 

 with the cavities of numerous processes of the body the so- 

 called feet which are the chief locomotive organs of the 

 adult. The radiating and circular vessels, with all their 

 appendages, constitute what is known as the "ambulacral 

 system ;" and in Astends and Echinids this remarkable system 

 of vessels remains in communication with the exterior of the 

 body by canals, connected with perforated portions of the 

 external skeleton the so-called "madreporic canals" and 

 " tubercles." In Opliiurids the persistence of any such com- 

 munication of the ambulacral system with the exterior is 

 doubtful, and still more so in Crinoids. In HolotJmrids no 

 such communication obtains ; the madreporic canals and their 

 tubercles depending freely from the circular canal into the 

 peri visceral cavity.' 



By Professor Wyville Thomson the larva of the Echinoder- 

 mata is termed the ' pseud-embryo ; ' since it leads a perfectly 

 independent existence, and the true Echinoderm is usually 

 developed out of a portion only of its substance. 



The Echinodermata are divided into seven orders, viz. the 

 Crinoidea, Gystoidea, Blastoidea, Ophiuroidea, Asteroidea, Echi- 

 noidea and Holoihuroidea. Of these, the first is almost extinct 

 and the two next are entirely so ; they are really the lowest 

 orders ; but their structure will be better understood if the 

 higher orders are considered first. 



