io THE ANTHOZOA 



sense cell. The fourth elements of the epithelial layer are the 

 gland cells, most abundant in the stomodaeum and on the mesen- 

 terial filaments. They are of two kinds, as shown in Fig. II. 

 7 and 8. 



The nervous layer of the ectoderm, shown in Fig. II. 1, con- 

 sists of a plexus of extremely fine fibrillae, giving in transverse 

 section a punctate appearance. In the depth of the fibrillar 

 layer are found, most abundantly at the bases of the tentacles, 

 bipolar and multipolar ganglion cells. These last lie directly on 

 the muscular layer, and are figured in Fig. I. 5, and in Fig. II. 

 5. The muscular layer lies directly on the mesogloea. It is 

 composed of very long and fine fibres, each of which bears about 

 the middle of its length a small mass of granular protoplasm, in 

 the midst of which lies the nucleus. 



The endoderm consists chiefly of epithelio-muscular cells, such 

 as are represented in Fig. II. 6. Each epithelio-muscular cell is 

 somewhat quadrangular in form in the extended condition of the 

 animal ; its free extremity is somewhat rounded and bears a single 

 long flagellum. Internally it rests upon a long and narrow 

 muscular fibre, which runs at right angles to it. The epithelio- 

 muscular cells of the endoderm contain yellow or green spherical 

 bodies which are symbiotic, unicellular algae, the so-called zooxan- 

 thellae or zoochlorellae. In addition nervous and glandular ele- 

 ments are found in the endoderm. 



The mesogloea of the Actinians consists of fine fibres imbedded 

 in a homogeneous matrix. Between the fibres lie numerous 

 small branched or spindle-shaped cells, the so-called connective 

 tissue cells. In many Actinians muscular elements are imbedded 

 in the mesogloea. 



The reader will be able to get a good general idea of the 

 histological elements of the Anthozoa by studying Figs. I. and 

 II. For further details he should refer to the work of 0. and R. 

 ' Hertwig (40). But it must be remembered that in the Anthozoa 

 histological differentiation reaches its highest point in the 

 Actinians. In the other groups the elements are simpler. 



The Anthozoa are divisible into two great sub-classes, sharply 

 marked off from one another by definite anatomical characters. 

 These are the Alcyonaria, sometimes called the Octactinia, and 

 the Zoantharia, sometimes called the Hexactinia. The last name 

 should be avoided. 



ALCYONARIA FIRST SUB-CLASS OF THE ANTHOZOA. 



The Alcyonarian zooid is distinguished by the following 

 characters : 



There are always eight, and never more nor less than eight 



