714 THE ANIMAL KINGDOM. 



and a few Hydrozoa, the latter in one Ctenophore, in the Anthozoa and 

 Hydrozoa. All are aquatic, and with few exceptions marine. The larva is 

 ciliated and, except in Ctenophora set free, as a rule, at an early stage of 

 development. 



With the absence of a mesoblast is correlated the absence of coelomic spaces 

 of all kinds, of a circulatory, specialised respiratory and nephridial systems. The 

 genital cells are sub-epithelial, or in Porifera mesoglaeal. 



The mesoglaea is a proteid substance, sometimes as dense as hyaline cartilage, 

 sometimes excessively soft and watery, e. g. it contains 95.392 / of water in Rhizo- 

 stoma Cuvieri (= Pilema pulmo). This percentage may be exceeded when the sea- 

 water is less saline than usual ; e. g. Aurelia aurita from Kiel has 97.90 %. Fibrillae 

 are very commonly present. They vary in character, size, mode of disposition, 

 whether reticulate, tangential, or vertical to the surface. They are probably derived 

 by condensation of the jelly : but in some instances very delicate fibrils have been 

 traced into continuity with the ecto- and endo-derm cells. As to mesoglaeal cells, 

 they may be wanting as in Craspedote Medusae : when present either simple or, as 

 in Ctenophora and Porifera, specialised in various ways. They may be derived from 

 the ecto- or endo-derm, or from both, and are sometimes set apart at an early period 

 as in Ctenophora (note 2, p. 720). The mesoglaea itself is probably derived in some 

 instances from the ectoderm, in others from the endoderm, e. g. in the taeniolae of 

 Acraspeda. 



An epithelium cell is one that is solely protective : an epithelio-muscular cell 

 is an epithelium cell with a basal muscular process, whereas a muscle cell is sub- 

 epithelial in position. Muscle cells may become inclosed within the mesoglaea by 

 the growth of the latter, and are then termed by Hertwig ' mesodermal muscles.' 

 The term ' neuro-muscular ' cell (Kleinenberg) has been applied to an epithelio- 

 muscular cell, but there is no reason to ascribe any special nervous function to the 

 cell portion of such a unit, any more than to the undifferentiated cell portion or 

 muscle-corpuscle of a striated muscle-fibre. The term has recently been used by 

 Korotneff in connection with certain remarkable branched ectoderm cells, the pro- 

 cesses of which are in continuity with the longitudinal ectodermic musculature of 

 some Siphonophora (Mitth. Zool. Stat. Naples, v. p. 235, PI. 14, Fig. 13). The 

 muscle substance is highly refractile, and is transversely striated in the subumbrellar 

 muscles of Medusae. 



A sense-cell is long, slender, provided with a cilium or stiff hair at the external 

 end, and prolonged basally into 2-3 fine filaments which are connected to nerve 

 fibres or processes of ganglion cells. A supporting cell is usually short and stoutish, 

 its basal end prolonged into filaments which probably enter the mesoglaea. Such 

 supporting cells occur wherever sense cells are present in numbers. In the eye- 

 specks their outer ends are pigmented. The other terms require no explanation. 



As to the endoderm, skeletal cells occur in the tentacles of many Hydrozoa, 

 cf. p. 329 on Fig. 7, PI. xiv, under C; skeletogenous cells are met with in some 

 Anthozoa Alcyonaria (?) ; sense and ganglion cells have been detected in the 

 prostomium of the Hydroid Eucopella and on the mesenteries of Hexactinians. 



For cnidoblasts, see p. 330, on Fig. 8, C, and for their contained nematocysts 

 p. 331, on Fig. 9, both PI. xiv. 



