Zoological Classification. 3G7 



is most interestingly displayed. The nervous system of 

 these creatures exhibits a decided advance, there being at 

 least three pairs of principal ganglia. Animals like the whelk, 

 the pteropod, and the cuttle fish, each carry the progress of 

 development still further, and prepare the way for the verte- 

 brate group. 



A great number of animals not yet spoken of must be 

 arranged somewhere in groups parallel with some of those to 

 which allusion has been made. Amongst these, the Echinodcr- 

 mata, of which the star-fishes may be taken as an example, form 

 a clearly distinct group. Animals of this kind begin life by 

 being hatched from an impregnated egg, and their first ap- 

 pearance is usually in the form of a free swimming ciliated 

 embryo, which undergoes changes of a remarkable kind. 

 Some echinoderms possess a continuous calcareous skeleton, 

 while others, like the sea- cucumber, are without it ; but have 

 their integument strengthened and defended by a multitude of 

 detached plates, or spicula. The larvae of these creatures soon 

 develope a distinct alimentary cavity, ' c divided into a well- 

 marked oral and cesophagal portion, a globular stomach, and 

 a short intestine terminating in an anal aperture." 



The life changes of the echinoderms are too complicated 

 and curious to be incidentally treated, we shall, therefore, in 

 this place pass them over, and observe that in adults the 

 mouth is found in the middle of a ' ' circular vessel" and " vessels 

 which radiate from the latter give off diverticula to communi- 

 cate 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 

 asterids (star-fishes), and in echinids (sea hedgehogs), 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 or tubercles." " fhe nervous system in all adult echino- 

 derms exhibits a ring-like, or polygonal gangliated cord, 

 situated superficially to that part of the ambulacral system 

 which surrounds the mouth, and sending prolongations 

 parallel with and superficial to the radiating ambulacral trunk." 

 Professor Huxley adds that the alimentary canal is less 

 dependent upon the form of the skeleton, and only in one 

 group, the Asteridea, shows anything approaching to a radial 

 disposition. The vascular system " is closely related to the 

 alimentary and ambulacral systems," but the details have not 

 yet been made out. 



Professor Huxley next considers seven groups, which he 

 has provisionally placed under the head Scolecida. This term 



