222 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



millimetres long, each of which bears only about three to 

 six pairs of lateral twigs, which are not so richly branched 

 as in the pinnate gill described above. This may be 

 called the dendritic type. 



Each gill is a hollow out-growth of the body wall 

 enclosing an extension of the coelom (fig. 36). The 

 wall of the gill is thin ; it is formed of an external cubical 

 or flattened epithelium, below which is a thin layer of 

 muscle fibres. The cavity of the gill is lined by the 

 coelomic epithelium, between which and the muscle layer 

 the branchial vessels are situated. The coelomic cavity 

 of the gill is crossed at intervals by muscle strands. The 

 gill is a contractile structure, and, as Milne Edwards 

 pointed out, the successive contraction of the branchiae, 

 which often proceeds regularly from behind forwards, 

 must exert a considerable influence in forcing the blood 

 into the efferent branchial vessels and into the vessels 

 of the body wall. 



The gills arise in the post-larval stage. One speci- 

 men, 3'9 mm. long, has already the full complement of 

 gills, but in other specimens examined the gills are not 

 formed until the animal is considerably longer. In the 

 fifteenth to the eighteenth chsetigerous segments of a post- 

 larval specimen, 4" 6 mm. long, the blood-vessels 

 immediately behind the notopodium form a loop pre- 

 paratory to the production of a gill, and there is a very 

 slight elevation of the body wall in this region. These 

 elevations soon become conical outgrowths, then digiti- 

 form and commence to branch. As Benham has pointed 

 out, the gills of Arenicola are from the first special 

 respiratory structures, and not as in Eunice, and some 

 other Polychaeta, modifications of dorsal cirri. There is 

 no trace of sense-hairs upon the epidermis of the develop- 

 ing gill, while such hairs are present in cirri. 



