142 
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. 
Kach gill is a hollow out-growth of the body wall 
enclosing an extension of the celom (fig. 56). 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 
celomic 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 branchiz, 
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, 39 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 chetigerous segments of a post- 
larval specimen, 46 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 Arentcola are from the first special 
respiratory structures, and not as in /unice, and some 
other Polycheta, modifications of dorsal cirri. There is 
no trace of sense-hairs upon the epidermis ot the develop- 
ing gill, while such hairs are present in cirri. 
