BRITISH NEMEKTEANS, AND SOME NEW BRITISH ANNELIDS. 405 



successive broods of young (from different individuals), which as minute white 

 specks darted about most actively. They did not crawl along the bottom, but, 

 like the young of Phyllodoce and other Annelids, swam freely throughout the 

 water after the manner of Infusorise, or danced to and fro like Ephemerse in the 

 air. Externally at this further stage of advancement they have still a coating 

 of very long cilia (Plate XIV. fig. 7), which serve as natatory organs, the tuft (c) 

 on each side being about thrice as long as the rest, while the long anterior 

 whip has disappeared. There are two large well-defined black eyes, no doubt 

 provided by nature for the exigencies of the youthful state, just as the young 

 of certain molluscs and Balani are similarly furnished. The mouth (a), the 

 oesophageal, and succeeding region of the digestive cavity are all richly ciliated. 

 The whole animal is soft and delicate, and none of my specimens survived this 

 stage. 



We have thus in Cephalothrioc a certain resemblance to the development of 

 M. Van Beneden's Polia involuta, already described (see p. 369), and the phases of 

 the growth of the present species likewise corroborate everything that has been 

 advanced in contradistinction to the interpretations of the Belgian author. His 

 views in regard to the scolex and proglottis receive no support from the foregoing 

 observations, for all the changes that occur are only the gradual and very per- 

 ceptible shedding of certain cilia, and the general advance of organisation as 

 shown by the differentiation of tissues and the appearance of pigment in the eye- 

 specks. The shedding of the long anterior tuft of cilia by the young Cephalothrix 

 has its analogue in the loss of the ciliated ring by the young Phyllodoce and others, 

 in the casting of the temporary bristles noticed by Busch and Leuckart* in the 

 young of a Nerine, and by M. de Quatrefages in the young stages of Hermella.f 

 I think there can be no doubt that the remarkable tuft of cilia which occurs in 

 the young Cephalothrix on each side of the snout, and which attains its full 

 development after the long anterior tuft has ceased to be conspicuous, is con- 

 nected homologically with the entrance to the cephalic sacs in the Ommatopleans 

 and the fissures in the Borlasians, as well as with the ciliated ring of Phyllodoce 

 above-mentioned. It is an embryonic type of a structure which disappears 

 entirely in the adult form. The delicacy of the young at the period of the full 

 development of the eye-specks is an interesting feature ; but it prevented my 

 observing their growth into perfect animals. 



Thus, so far as development goes, Cephalothrix is nearly allied to the Omma- 

 topleans, especially to Tetrastemma variegatum, Polia involuta, and probably to 

 others of the group not yet investigated ; while, in the structure of its digestive 

 system, circulatory apparatus, and the unarmed proboscis with its bridled sheath, 

 it leans rather towards the Borlasians. Prof. Keferstein in his proposed classi- 



* Ann. Nat. Hist. 2d ser. vol. xvi. p. 259, pi. vii. 

 f Annales des Sc. Nat. 3d ser. torn. x. 1848. 



VOL. XXV. PART II. 5 L 



