310 DE W. CARMICHAEL M'lNTOSH ON THE STRUCTURE OF THE 



structure of Ommatoplea (his Polio) with Borlasia, and partly from incorrect 

 observations. He refers to the cells or areola? of the integument as "simples 

 vacuoles ovo'ides ou arrondies," which refract light, takes no note of their con- 

 tents, and apparently considers them empty. His separation of the skin into two 

 layers, the exterior composed of smaller, the interior of larger cells, is not evident 

 in Ommatoplea. Smaller cells sometimes do occur towards the ciliated surface. 

 but the entire integument-proper is continuous as a single layer. The only 

 representative of his "fibrous" layer, which is described as lying within the 

 former, is our structureless basement-layer. Dr Schultze* figures a small por- 

 tion of the skin of his Tetrastemma obscurum, showing a series of large cells under 

 the epidermis, with a few granular bodies interspersed, but the view is diagram- 

 matic. Prof. Keferstein's observations on the cutaneous and muscular struc- 

 tures apply almost entirely to our Porlasians. 



Muscular Layers of the Body. — A very distinct belt of circular muscular fibres 

 (Plate IV. fig. 2, c) occurs next the basement-layer of the cutis. They (the fibres) 

 are compact throughout, and less bulky than the next coat, with which their fila- 

 ments do not mix. The succeeding layer (d) forms a powerful wall of longitudin al 

 muscular fibres, which, in transverse sections, is generally somewhat crenated on 

 its inner border, and fasciculated throughout. The interfascicular substance is 

 transparent and structureless, and evidently as mobile and contractile as the 

 fibres themselves. Numerous fibrous bands stretch from the inner surface in 

 connection with the various contents of the body. The muscular coats in Tetra- 

 stemma are formed on the same plan as the foregoing. The appearances of these 

 muscles in transverse section resemble those recently given by Professor Kol- 

 liker of the muscles in crabs.f Thus there are only two distinct muscular coats 

 of non-striated fibres around the body of the Ommatopleans, making an essential 

 difference in this respect between them and the Borlasians, to which (latter) 

 previous observers have for the most part confined their investigations. 



M. de Quatrefages describes the muscular coats both in Borlasia and Nemertes 

 (specially instancing Nemertes balmea, our Ommatoplea gracilis), as consisting of 

 " external longitudinal and internal transverse" fibres. In Ommatoplea, as just 

 described, it is exactly the reverse, the circular fibres being external, and the 

 longitudinal internal. He also speaks of another layer, within the internal, as 

 forming an aponeurosis, apparently referring to the fibrous prolongations from 

 the internal or longitudinal coat, j Thus Sig. Delle Chiaje, instead of being in 

 error, as averred by M. de Quatrefages, is correct in stating that the external 

 coat is circular, and the internal longitudinal. Physiologically, it is certainly a 



* Beitrage zur Naturges. der Turbellaiien, tab. vi. fig. 4. 

 f Zeitsch. fur wiss. Zool. bd. xvi. 1866, p. 375 



+ Van Der Hoeven, apparently from following M. de Quatrefages, makes the same errors 

 — Hatidbuch der Zoologie, vol. i. p. 212. 



