BRITISH NEMERTEANS, AND SOME NEW BRITISH ANNELIDS. 401 



flask, and the scattered cells and granules, are portions of the discarded external 

 covering of the embryo ; and it is to be observed that the cilia on this texture 

 are, if anything, longer than those on the free young animal, though their motion 

 is less vigorous. The " cells" of which this rejected covering is made up are 

 entirely of a fatty nature (Plate XI. fig. 10)— in short, an aggregation of fatty 

 granules, with an oil-globule or two, and capable of changing form accordingly. 

 It is a fact that this debris after a time quite disappears from the flask, and 

 therefore it probably acts as nourishment for the young (being swallowed by the 

 mouth, as in the case of the embryo of Purpura lapillus) just as the yolk-sac, 

 by a different mode, does in other animals. In escaping from the flask, the 

 young animals, in many cases, seem to have thrust themselves along the narrow 

 apex, dilating it and bursting through. For a considerable time afterwards they 

 crawl about in swarms amongst the gelatinous mucus, so that the latter has a 

 curious aspect, being filled, in addition, with the transparent flasks from which 

 they have escaped, and a few undeveloped ova. Moreover, it is a common prac- 

 tice for the adult animals to crawl through these masses, and several are gene- 

 rally coiled in proximity. The number of undeveloped ova is extremely small, 

 showing how easy it is to rear these animals, even with very limited supplies of 

 fresh sea-water. 



The foregoing development is thus much less complicated than the remark- 

 able evolution of the Nemertean worm, called Alardus caudatus, Busch., from 

 Pylidium gyrans, as described by J. Muller* This form would seem to be allied 

 to Sir J. Dalyell's Stylus (Micrura), since it is furnished with a process poste- 

 riorly ; and the author states that most examples are eyeless. Leuckart and 

 Pagenstecher| have also recorded another species of Pylidium, and the develop- 

 ment of the Borlasian worm therein ; and they remark that the mouth of the 

 worm is in connection with that of the Pylidium — indeed the organ in the latter 

 opens into it— a statement verified in the same volume of the " Archiv" by 

 Krohn. 



The young Borlasians, at the stage previously mentioned, are visible to the 

 naked eye as small elongated worms, somewhat tapered at both ends, pale, or 

 rather translucent in front, and opaque- whitish posteriorly (Plate XIII. fig. 5), and 

 in structure now closely approach the adult. The whole surface of the body is 

 richly ciliated, the cilia being especially active in the cephalic fissures, and still 

 more so at the openings of the cephalic sacs. The ganglia are indicated by a 

 paler space (h) on each side, but their actual outline is indistinct. There are in 

 all cases at least two well-marked eyes. The cephalic sacs (m) are large and 

 well defined, indeed very much larger proportionally than they are in the adult : 

 and from their present position with respect to the ganglia, demonstrate the true 



* Archiv fur Anat. &c. 1854, p. 75, taf. 4. f Archiv fur Anat. 1858. 



VOL. XXV. PART II. 5 K 



