110 THE ANATOMY OF IXYERTEBRATED ANIMALS. 



Myxospongim being altogether devoid of skeleton ; the Cal- 

 cispongice possessing calcareous spicula, but no fibrous kera- 

 tose skeleton ; and the Fihrospongim having a fibrous skele- 

 ton, and (usually) spicula of a silicious nature. To these it is 

 probable that the Clionid(J& must be added, as a fourth type, 

 devoid of a fibrous skeleton, but possessing silicious spicula 

 of a very peculiar kind, by the help of which they are able 

 to burrow parasitically in the shells of mollusks. Finally, 

 Haliphysema and Gastrophyseina appear to be even simpler 

 than the Myxosponglm. 



The division of the Myxospongice contains only the ge- 

 latinous Halisarca, The Calcispo?igiCB, in addition to the two 

 families of Asco7tes and Leiicones^ already referred to, include 

 a third — the Sycones^ which are essentially composite As- 

 coyies. The I^ibrospongice present a great diversity of form 

 and structure. They may have the form of flattened or glob- 

 ular masses, arborescent, tree-like growths, flagellate expan- 

 sions, or wide or deep cups. The sponge of commerce de- 

 rives its value from the fact that its richly-developed fibrous 

 skeleton is devoid of spicula. On the other hand, in such 

 sponges as Hyalonema and Euplectella^ the silicious spicula 

 attain a marvelous development and complexit}^ of arrange- 

 ment. In the latter genus, they form a fibrous network with 

 regular polygonal meshes. These appear to be the repre- 

 sentatives of the Ventriculites^ which were so common in 

 the seas of the Cretaceous epoch. 



Sponges abound in the waters of all seas, but Spongilla 

 is the sole fresh-water form. Clionidm existed in the Silu- 

 rian epoch, but the most plentiful remains of sponges have 

 been yielded by the chalk. 



The CoELE^'TERATA. — This group of the Metazoa contains 

 those animals which are commonly known as Polyps, Jelly- 

 fishes, or Medusce^ Sea-anemones, and Corals. They exhibit 

 two well-marked series of modifications, termed the Hydrozoa 

 and the Actmozoa, 



The Hydrozoa. — The fundamental element in the^ struct- 

 ure of this group is the Hydranth., or Polypite. ^ This is es- 

 sentially a sac having at one end an ingestive or oral open- 

 ing, which leads into a digestive cavity. The wall of the sac 

 is composed of two cellular membranes, the outer of which is 

 termed the ectoderm^ and the inner the endoderm^ the former 

 having the morphological value of the epidermis of the higher 



