300 On the Nephridia &c. of Avenicola. 



the brain is continued into one or two small protuberances, 

 which project into the space lying above the mouth. 



At the sides of the head immediately under the epidermal 

 cells lie the otocysts, two in number. Each consists of a 

 central cavity containing small, irregular, transparent bodies 

 — the otoliths. There is a single layer of cells surrounding 

 this cavity with very small cilia. This layer of cells, though 

 quite distinct, is not so clearly marked off from the neigh- 

 bouring tissue as in the adult, nor is it so compact or regular. 

 In the adult the otocyst lies within the muscles of the skin 

 and quite free from them ; in this case it is close to the 

 epidermis and is surrounded by the mesenchyme-cells. This 

 mesenchyme-tissue is continued along the body-wall, and 

 makes it difficult to distinguish between the longitudinal and 

 circular muscles. The external layer of glandular cells is 

 more highly pronounced than either of these muscle-layers. 



The stomach — a long tubular organ — is lined by a single 

 layer of cells, which are smaller than those of the gullet and 

 other parts of the alimentary tract, and do not stain so deeply. 

 The walls of the gut diverticula, which lie dorsally to the 

 stomach, have small glands upon them which project into the 

 lumen. These glands have a central vascular core with cells 

 clustered round. The first part of the intestine consists of 

 large cells, which are also glandular in their nature. 



References. 



(1) Benham. — " The Nephridium of Liimbrictis, with Remarks on 



the Nephridia in other Chfetopoda," Quart. Journ. Micr. Sci. 

 vol. xxxii. 



(2) Benham. — " The Post-larval Stage of Arenicola marina" Journ. 



Mar. Biol. Assoc. 1893. 



(3) CosMOVici. — '' Glandes Genitales et org. segmentaires des Ann^l. 



Polychetes," Arch. d. Zuol. exp. et gen. viii. 1879. 



(4) Cunningham. — " On some Points in the Anatomy of Polychcetes" 



Quart. Journ. Micr. Sci. vol. xxviii. 



(5) Ehlers. — " Sur Kentuiss von Arenicola marina,^^ Nachrichten von 



der lioniglichen Gesellschaft . . . . zu Gottingen. 1892. 



(6) VoGT and Yung. — ' Anatomie comparee pratique.' 1888. 



EXPLANATION OE PLATES XIU.-XV. 



Figs. 1 & 2. Nephridia in position : 



(1) where nephrostome is parallel to the long axis of the body ; 



(2) where nephrostome is inclined to this long axis. 



a, fringed dorsal lip of nephrostome, the arrow points towards 

 internal opening of nephridium; b, thin, almost transparent part 

 of the neck of nephridium ; c, dilated posterior portion, which 

 contains amoeboid cells similar to those found in body-cavity, 

 also a brown gi-anular substance which seems to be the product 

 of excretion from the cells in the central part of nephridium ; 



