112 LECTURE X. 



in size under the influence of the light and warmth, and the abundant 

 food, which result from the stimulus of the rays of the summer sun 

 upon the surface of the ocean. 



In comj^aring the several stages in the very interesting development 

 of the CyancBci aurita to the Infusoria and Polypes, it must be under- 

 stood that such comparisons are warranted only by a similarity of 

 outward form, and of the instruments of locomotion and prehension. 

 The essential internal organisation of the persistent lower forms of 

 the Zoophyta is entirely wanting in the transitory states of the higher 

 ones. A progress through the inferior groups is sketched out, but 

 no actual transmutation of species is effected. The young Medusa, 

 before it attains its destined condition of maturity, successively 

 resembles, but never becomes, a Polygastrian, a Rotifer, and a Bryo- 

 zoon. 



LECTURE X. 



ECHINODERMA. 



The soft and gelatinous Radiaries have often baffled the anatomist 

 by the seeming simplicity and uniformity of their texture ; the 

 harder, spine-clad, or Echinodermal species, perplex the most patient 

 and persevering dissector by the extreme complexity and diversity of 

 their constituent parts. 



This class of animals, the organisation of which I shall endeavour 

 to explain in the limits of the present Lecture, includes species in 

 which the form is most strictly or typically radiate : in it, also, the 

 Zoophyta of Cuvier attain their highest conditions of organisation. 

 With a radiated filamentary system of nerves, we find not only a 

 distinct abdominal cavity with an alimentary canal suspended therein 

 by a vascular mesentery, and having a distinct anal outlet, but like- 

 wise a large and well-defined respiratory organ. This organ, how- 

 ever, may be regarded as the exceptional condition of the radiated 

 type of structure, and is found only in the highest and aberrant forms 

 of the present class, which indicate the transition from the Echino- 

 derms to the Annelides. At the opposite extreme of the Echinoderma, 

 the digestive sac (^Jig- 64-. «), though suspended freely in an abdominal 

 cavity, has yet but one aperture common to the reception of food 

 and the ejection of excrement. These anenterous Echinoderms belong 

 to the family (^S teller ida?^, in which the radiated form is most complete 



