Fissiparous Species of Tubicolar Annelid. 119 



pairs of close-set, double pedal processes. The lower portion of 

 each process forms a mere transverse ridge, beset with the 

 peculiar hooks to be described by and by ; the upper pro- 

 cess, on the other hand, is conical, and is provided with 

 elongated setae. The most striking feature of the thorax, 

 however, consists in the peculiar membranous expansion, (b) 

 which, arising as a ridge upon each side of what might 

 be termed the nuchal surface of the animal, and attached 

 to the sides of the thorax, above the bases of the feet, runs 

 down to terminate on the ventral surface, behind the last pair 

 of thoracic appendages. From this origin it extends as a wide 

 free membrane beyond the setse, forming an elegant collar 

 around the head, on whose ventral surface the expansions of each 

 side unite, and form a wide refiexed lobe (fig. 4, g), while poste- 

 riorly they remain separate. To the thorax succeeds what may 

 be called the abdomen, which is much longer than the other 

 regions of the body ; and is, besides, distinguished from them 

 by the imperfect development of the feet, and the paucity of 

 the setae and hooks. In this, and in the caudal portion of the 

 body, the relative position of the hooks and setae is the reverse 

 of what it is in the thorax, the former being superior, and the 

 latter inferior. * 



The caudal portion of the body is short, and wider than 

 the abdomen. Its rings are close-set, with well-developed 

 hooks and setae, and it is terminated by two conical papillae 

 between which the anus is situated. There are not less than 

 50 rings in the whole body. Cilia could be detected in active 

 motion on many parts of the external surface, on the bases of the 

 feet, on the rudimental tentacles, and scattered in tufts over 

 the whole surface of the thoracic expansions. 



Having thus sketched its external character, I will now 

 pass to the minuter features presented by the organization of 

 the animal. 



Branchial plumes. — The principal mass of these organs is 

 formed by a clear, firm, supporting axis, so marked transversely 

 as very closely to resemble the chorda of an Amphioxus. The 

 lower end of this axis terminates by a somewhat pointed ex- 



* According to Grube, this is the case in all the Serpulacea. See his most 

 excellent work — " Die Familien der Anneliden." 1851. 



