BRITISH NEMERTEANS, AND SOME NEW BRITISH ANNELIDS. 309 



toplea purpurea and 0. gracilis the cells of the skin are much smaller than in 

 0. alba. In 0. gracilis, indeed, the skin resembles microscopic mosaic work, 

 from the granules and plaits in each space or cell. 



The function of this elaborate glandular arrangement is doubtless the secre- 

 tion of the abundant mucus so characteristic of these animals, and which is often 

 of a most tenacious description. I have seen a specimen rapidly form a sandy 

 investment by this means, when placed in a vessel containing a little sand ; and 

 whether the sand particles simply adhered to the gelatinous mucus by accident 

 or not, the animal took full advantage of the protection. The same habit is exten- 

 sively followed by the Ommatopleans of our southern shores, apparently to protect 

 themselves from the increased danger of desiccation. On placing a living speci- 

 men on a glass slip, and causing it to emit some mucus, the secretion proved to be 

 a minutely granular fluid, intermingled with a few larger corpuscles. The silky 

 sheaths formed by Tetrastemma variegatum and others are well-known examples 

 of this cutaneous secretion. The tube constructed b)^ Polia involuta, Van Ben.,* 

 is the densest yet seen, and it has an areolar aspect, from the granules or globules 

 being set in a hyaline matrix, sometimes at considerable intervals from each other. 

 Moreover, when viewed in profile, these globules are found to be elevated above the 

 external surface, like a series of low pale warts. M. Beneden says it is simply 

 tesselated. The tube is attached to the hairs of the abdominal feet of female 

 crabs (C. maenas) bearing ova, and is evidently of intrinsic importance to the 

 species, both as a protection against injury and desiccation. That some of the 

 characters of this group of worms are due to the thick and soft cutaneous layers 

 is demonstrated by the appearance which they present when such are removed, as 

 by improper preservation. Two specimens of 0. pulchra, dredged off the Hebrides 

 by Mr Jeffreys, were in this condition ; and as the proboscis had been thrown off 

 in the one first examined, it appeared like a new type of non-bristled worms, 

 characterised by the simple arrangement of its digestive system, and its glisten- 

 ing and elastic investment, so different from the dull, whitish, and non-elastic 

 covering of an ordinary preparation.! Another interesting feature in regard to 

 the skin of the Ommatopleans (in common with the Borlasians), is the reaction 

 which ensues on testing with litmus-paper. In this group an acid reaction occurs 

 in 0. alba, 0. melanocephala, and 0. gracilis; while, on the other hand, a reaction 

 not less distinctly alkaline characterises 0. purpurea and 0. pulchra. 



M. de Quatrefages' description of the tegumentary structures differs mate- 

 rially from that just given, a discrepancy arising partly from his confounding the 



* Nemertes carcinophilus, Kolliker. 



\ The comparison of the external tissues of certain remarkable processes, occurring on a new 

 Annelid from the Gulf of Suez, to the Nemertean skin, as described by M. le Dr Leon Vaillant 

 in the " Ann. des Sc. nat " for 1865, is certainly far fetched and unlikely. The processes referred to 

 are considered buds, but they seem to me to be no more buds or parasites, than the processes on the 

 long tentacles of our British Moea >nirabilis. 



VOL. XXV. PART II. 4 K 



