THE ANTHOZOA 



be employed. The position and relations of the mesenteries in an 

 ideal Anthozoan zooid may be understood by reference to Fig. I. 

 1 and 2. Each mesentery is attached by its upper margin to 

 the peristome, by its outer margin to the body wall, and by its 

 lower margin to the basal disc. Typically it is attached by the 

 upper part of its inner margin to the stomodaeum, but below 

 the stomodaeum it ends in a free edge, on which is placed a 

 thickening known as the mesenterial filament. A mesentery 

 consists of a middle layer of mesogloea, covered on both faces with 

 a layer of endoderm. The mesenterial filament is often ectodermic 

 in origin. The gonads or reproductive organs are borne on the 

 mesenteries, the germinal cells being derived from the endoderm. 

 The Anthozoa, like all the other Coelenterates, are provided with 

 special offensive weapons in the form of cnidae or nematocysts. 

 The nematocysts of the Anthozoa are in many cases rendered 

 complex by the presence of numerous spines on the whole length 

 of the eversible thread. In the nematocyst of Coryiiadis, shown 

 in Fig. I. 3, the spines are arranged in a double spiral. The 

 nematocysts of the Alcyonaria, on the other hand, are generally 

 simple, small, and devoid of spines (Fig. IV. 8). 



The histology of the Anthozoa has been studied with some 

 care in the case of particular groups, especially in the Actiniae by 

 0. and R. Hertwig (40). In these forms the ectoderm consists of 

 three not very clearly defined layers : (a) The epithelial layer ; (b) 

 the nervous layer ; (c) the layer of muscular fibres. 



Four elements are distinguishable in the epithelial layer. The 

 preponderating elements are the elongate, almost thread-like, 

 ciliated cells, whose characters may be studied in Fig. II. 1, 2, 

 and 3. 3 represents cells from the tentacle of Ammonia sulcata, 

 and it will be observed that each bears a tuft of fine and short 

 cilia at its broader peripheral end. 2 represents partly isolated 

 cells from the ectoderm of the body wall of Corynactis viridis. 

 In this case each attenuated cell bears a single flagellum at 

 its outer extremity. Similar cells are found on the mesenterial 

 filaments of Sagartia parasitica and other forms. 



Amongst the ciliated epithelial cells are found sense cells, one 

 of which is shown in Fig. II. 3. They occur chiefly on the 

 peristome and the tentacles. Each sense cell bears a single stiff 

 hair at its peripheral extremity, and internally ends in several 

 very fine varicose fibrillae, which are continuous with the fibrils 

 of the nerve layer. 



The third element of the ectoderm is the cnidoblast shown in 

 Fig. II. 4. Each cnidoblast forms, as an entoplastic product, a 

 single nematocyst. It is provided at its peripheral extremity 

 with a single stiff hair or cnidocil, and internally it ends in a 

 fibre which branches to form numerous fibrillae like those of a 



