BRITISH NEMERTEANS, AND SOME NEW BRITISH ANNELIDS. 403 



in November. His figure of the spermatozoa of this species* is not correct, as 

 no tails are present, and he describes them as simple rods. He makes the interest- 

 ing statement,! that in the same animal he found the embryos in some ova covered 

 with vibratile cilia while yet in the body of the parent, while others were only 

 fecundated during or after deposition. The development of the curious form 

 described by Mr Alex. Agassiz,| which, commencing with an oral and anal 

 circlet of cilia, gradually looses these and two short antennae which subsequently 

 appear, and assumes the form of Nareda (Girard) with two eyes, shows that the 

 type of growth is different from that of any British species yet observed. The 

 opening of the mouth (to all appearance) behind the ganglia points to some affinity 

 with the Borlasians ; but the absence of so important an organ as the proboscis, 

 which very soon becomes conspicuous in all the young British forms, again leaves 

 us in doubt as to its actual position. The young Nemertean described by Dr 

 Busch,§ under the name Alardus caudatus, would seem to have some relation to 

 Stylus (Micrura), since Jt possesses a very distinct tail. The apparent segmen- 

 tation of the latter, however, is characteristic. 



In Cephalothrix (Astemma) the ova and spermatozoa are developed in a dense 

 series of sacs (that give the animal a transversely barred aspect), which com- 

 mence a short distance behind the mouth and continue nearly to the tip of the tail. 

 The males are distinguished by their somewhat paler aspect when their reproduc- 

 tive organs are fully developed, viz., towards the end of January and during the 

 subsequent spring months. The spermatozoa (Plate XI. fig. 3) consist of short 

 flattened spindles with rounded instead of pointed ends, that to which the tail is 

 attached being somewhat smaller than the other. In swimming about the two 

 ends appear as clear dots. Though the animal is extremely elongated, the bodies 

 or " heads" of the spermatozoa are comparatively short. The body of the female, 

 with matured ova, presents a duskier or slightly fawn-coloured aspect, the ova, 

 under slight pressure in the living animal, being arranged in dense transverse rows 

 in each ovary. The total number of ova produced by a single female must be very 

 great. In transverse sections they are seen to occupy a large ovoid space on each 

 side of the alimentary canal, upwards of twenty ova — very prettily arranged in a 

 concentric manner — occurring in a single thin slice. The space of the digestive 

 canal in these preparations had thus assumed the form of the letter x, the walls 

 approaching each other in the middle, but diverging superiorly and inferiorly ; 

 while a wedge-shaped fold from the dorsum below the proboscis, and another from 

 the ventral surface, completed the resemblance. This was the more marked, if the 



* Op. tit. pi. i. fig. 13. 



f Ojj. tit. p. 13. — "La vesicule germinative ayant disparu, le vitellus s' organise, et, avant la 

 ponte, nous avons trouve des embryons couverts de cils vibratiles." 



I Ann. Nat. Hist, 3d Ser. vol. xix. 1867, pp. 208-214, pi. v. figs. 3-17. 



§ Beobacht. iiber Anat. u. Entvvickelung einiger Wirbellos. Seeth. Berlin, 1851, p. Ill, taf. xi. 

 fig 8. 



