52 THE APODOTJS HOLOTHUEIANS 



white spots on the skin. The irregularly branched forms are sometimes called 

 rosettes (Plate IV, figs. 18 and 23). In size the miliary granules vary from 

 3 to 30 /*. 



3. The circular muscles of the body-wall of all Synaptids run completely 

 around the body-cavity, just without its lining epithelium. In most species 

 the layer is of uniform thickness and continuity in all parts of the body, ex- 

 cept that it becomes somewhat better developed near the cloacal opening, for 

 which it forms a sphincter muscle. It may perhaps also thicken on or near 

 the oral disc. In Acanthotrochus, however, the circular muscles occupy only 

 the interradial portions of the body-wall, except at the ends of the animal, 

 where they are continuous, as in other Synaptids. 



4. The longitudinal muscles of the body-wall are confined to the radii, 

 where they project more or less markedly into the body-cavity, the epithelial 

 lining of which covers them. They are usually single folds or strips, attached 

 anteriorly to the calcareous ring, and posteriorly thinning out and disappear- 

 ing near the cloacal opening. They are generally most compressed anteriorly, 

 and become flattened near the middle of the body and imperfectly cylindrical 

 at the posterior end. In some cases the anterior part of the muscle may be di- 

 vided into an outer and an inner portion, and in such cases the inner half 

 is called a retractor muscle, and is regarded as homologous with the retractor 

 muscles of Dendroohirotse ; but the constant presence of such muscles in any 

 Synaptid is open to doubt; they are said to be characteristic of Euapta lappa, 

 Synapta maciilata, and Chiridota rotifera, but although I have examined these 

 species, macroscopically and microscopically, both living (except maculata) and 

 preserved in alcohol, I have never found the slightest evidence of a retractor 

 muscle. Those of Chiridota rotifera are said to be very short and joined by 

 connective tissue to the longitudinal muscle. In this statement lies a probable 

 explanation of the whole difficulty; the anterior portion of the longitudinal 

 muscles, being strongly compressed, may divide more or less incompletely in 

 a plane parallel to the body-wall, and where the division is only partial such 

 an arrangement as is reported for Chiridota rotifera would arise, while if the 

 division were completed it would result in the arrangement said to occur in 

 C. discolor. Now, as I have examined specimens of both rotifera and discolor 

 and have failed to find any trace of a separate retractor muscle, I am con- 

 vinced that the matter is one of individual diversity, or perhaps correlated 

 with age. At any rate, the presence or absence of retractor muscles is a char- 

 acter upon which only the slightest value can be placed for purposes of classi- 

 fication. 



5. The innermost layer of the body-wall is a simple, epithelial layer, which 

 covers all of the surface bordering on or projecting into the body-cavity. It 

 consists of polygonal cells similar to the ordinary epithelial cells of the outer 



