128 
THE YOYAGE OF H.M.S. CHALLENGER. 
running along the genital furrow in its immediate neighbourhood towards the frontal 
shield, within which it forms an arch, and leaving it in the region below the right rhino- 
phore, enters the body cavity ; now this long free portion of the duct is somewhat thicker, 
and forms a coil in which the two parts, the prostatic and muscular, can often be easily 
recognised by their colour. To the last part of the vas deferens is attached a strong 
retractor muscle, and the vas deferens is then continued into the longer or shorter 
(when retracted) sac-shaped penis, which opens into the male genital cleft . 1 In most 
Onchidiadse the last part of the vas deferens is lined with a strong cuticle, which, as in 
the Doriopsidse, Phyllidiadae, and other Nudibranchiata, has longitudinal rows of small 
hooks ; the anterior portion of this last part can be everted. In many Onchidiadse 2 there 
opens near the penis a very long coiled glandula hastatoria, which is prolonged in front 
into a spindle-shaped or sausage-formed ampulla, opening on to the male genital cleft at 
the side of the penis by its special duct and the straight long dart at its end. 
Concerning the development of Onchidium not much was known until recently. 
Stoliczka observed that the young animals live massed together in deep earth-holes, 
and remarked that they perhaps had a direct development without larva. Semper 3 endea- 
voured, but in vain, to find the eggs. Joyeux Laffuie 4 finally succeeded in tracing the 
development of Onchidium celticum. 
Onchidium appears to be amphibious, inasmuch as it is found on those parts of the 
shore where there is a regular ebb and flow . 5 According to Semper 6 the function of the 
dorsal eyes is to protect the animal from its (presumed) chief enemy, Periophthalmus ; 
immediately it sees one of these approaching, it draws its body together and squeezes 
out a secretion from abundant cutaneous glands. 
Onchidium is mainly an inhabitant of tropical or sub-tropical regions ; from the 
Mediterranean only one species ( Onchidium parthenopeium, d’Ch.) is known, and a 
very similar ( Onchidium celticum, Cuv. ; Onchidium boreale, Dali) from the northern 
part of the Atlantic. The different “ species ” agree very much in the outer form, and 
most of the species described by different authors will not be recognised with certainty. 
In recent times Semper has observed that certain parts of the genital apparatus afford 
useful systematic characters. The division of the Family into the genera Onchidium, 
Peronia (Blainville), and Onchidella (Gray) cannot be retained. Stoliczka 7 first clearly 
showed this. Semper 8 divides the Onchidiadse into Onchidium proper and Onchidella 
1 Semper, loc. cit., p. 254. 2 Semper, loc. cit., p. 254. 3 Semper, loc. cit., p. 488. 
4 .Joyeux Laffuie, Organisation et diveloppement de le l’Oncidie, Oncidivm celticum, Cuv., Archives de Zool. expdr., 
t. x., 1882, pp. 1-159, pis. xiv.-xxii. 
6 Jhering, Ueber die system. Stell. von Peronia, 1877, pp. 9-15. — Joyeux Laffuie, loc. cit., p. 237. 
0 Loc. cit., pp. 30-32. 
7 Stoliczka, The Malacology of Lower Bengal, I., On the genus Onchidium, Journ. of Asiat. Soc., vol. xxxviii. 
2, 1869, pp. 100, 101. 
3 Semper, loc. cit., Erganzungsh., 1877, p. 40 ; Heft v., 1880, p. 254. 
