108 



ZOOPHYTES. 



highest developement in the Echinoderms. Among zoophytes, the 

 Hydra forms the first step upward, in which the digestive cavity is a 

 mere sac, which will work equally well turned inside out, and the 

 mode of reproduction is extremely simple. From this group, we 

 pass to the Actinia, in which there is a distinct stomach and a series 

 of fleshy lamellfe around the internal cavity, — the first rudiments of 

 an isolation of the functions of digestion and generation, — but the 

 circulating fluid is only the elaborated chyle, mingled with more or 

 less water from without. A step farther, and we find the develope- 

 ment of separate organs for the functions of the liver and of genera- 

 tion in the Echinoderms, and in some species a circulating system. 



Whether the Acaleph* or Zoophyta are highest in the scale, we do 

 not intend to indicate above. The young of some zoophytes are 

 acalephs in form, even to organs of sight, and the young of some 

 acalephs are much like polyps : moreover, the adult acaleph is often 

 quite analogous in its radiately subdividing alimentary channels, to 

 the same in the Fungia. 



Through the Bryozoa, the infusoria are connected with the Tuni- 

 cata and the other Molluscs ; and through the Rotifera and Entozoa, 

 they connect with the Articulata, thus passing each way, out of the 

 true Radiate type, into that which characterizes the higher sub- 

 kingdoms. The Bryozoa, Rotifera, and Entozoa, may be arranged 

 in the sub-kingdom Radiata, or with the Mollusca and Articulata, 

 whose types of structure they exhibit, though under a Radiate form. 



The Echinoderms, although so strikingly peculiar in some species, 

 — the Echini, — yet, through the Holothurife, bear closely upon the 

 Articulata ; while the Acalephs incUne toward the Mollusca. 



The animal kingdom is throughout a network of affiliations, yet 

 there are main trunks and larger branches, to which the smaller 

 anastomosing ramifications are subordinate. Systems of 3s and 5s, 

 look pretty to the mathematical eye, and have some foundation in 

 nature ; yet, in tracing affinities, it is better first actually to ascertain 



Crustacea, the same or analogous organs, together with the mouth, hecome lateral, owing 

 to the developement and projection of the cephalic ganglion — and its accompaniments 

 constituting the head — beyond to one side of the circular series of the natatory plates ; 

 these natatory plates, about the Rotifer's mouth, becoming, at the same time, the branchia; 

 and the attached maxillary organs about the Crustacean's mouth. The transition from 

 the Crustacean to the Radiate type is also shown in the passage of the Caligi into the 

 Epizoa. This subject admits of a long series of illustrations, which are reserved for 

 another place. 



