The Pluteus oj Laganum sp. 



135 



encircling a region slightly anterior to the center, beneath which lies 

 the ring of the hydroccele. Here, then, the adult mouth is evidently 

 to form. 



The wall of the amniotic cavity is composed of 2 layers of tissue 

 which are continuous with each other around the external opening. 

 They consist of an inner layer of large irregular cells, and an outer 

 epithelium of cylindrical cells with darkly staining nuclei, usually 

 situated well back toward the base of the cell. In the inner layer the 

 cytoplasm is clear, and takes the form of a loose, irregular meshwork ; 

 in the outer it is opaque and is crowded with pigment granules exterior 

 to the row of nuclei. Occasionally, however, large areas may be found 

 in the outer layer, within which the cytoplasm contains many irregular 

 vacuoles, indicating that possibly the wall of the amniotic cavity is 

 beginning to break down. 



FIG. 17. Transverse sec- 

 tion through anterior part 

 of body, showing two 

 anterior primary tenta- 

 cles, a.c., anterior en- 

 teroccjele; P, I 3 , lumen of 

 lobes 2, 3, of hydrocoele; 

 sp, spine; t 2 , t 3 , tentacles 

 2, 3. X 233. 



17 



Histologically, the spines also are composed of 2 layers of tissue. 

 The inner central one forms a core in which the cytoplasm possesses a 

 large irregular meshwork. In the outer the cytoplasm is denser and 

 crowded with pigment granules, and the nuclei stain more heavily 

 with the hematoxylin. Between these two layers lie well-developed 

 longitudinal bands of muscle. Within the spines were originally the 

 skeletal supports described above, which have been destroyed in these 

 specimens by the fixing fluid. 



THE ENTEROCCELES. 



Within the embryo, transformation from larva to adult is already 

 beginning to take place. The gut has lost its opening to the exterior, 

 the left posterior enteroccele is definitely constricted from the anterior, 

 and the hydrocoele with its 5 primary tentacles is already in an ad- 

 vanced stage of development. 



