BRITISH NUDIBRANCHIATE MOLLUSCA. 27 



alluded to. This latter (PL 45, figs. 8, 9,) is about one sixteenth of an inch in length, depressed, 

 with the thoracic shield quite distinct from the abdomen, and composed of only one 

 segment, of an irregular oval or sub-quadrilateral form, broadest behind : a projection in front 

 covers the head. The abdomen is narrow ; it tapers a little, and is formed of four or five 

 segments. The tail is bilobed, each lobe bearing a single stout hair. The mouth is situated 

 below, and has on each side a hook, supported on a flexible base ; the head is likewise furnished 

 with a pair of articulate antennae. There are three pairs of rather feeble thoracic feet, the 

 last articulation bearing a slender hook. 



Whether these two forms are distinct species, which from their great dissimilarity might 

 be supposed to be the case, or are the opposite sexes of the same species, cannot at present 

 be determined; but their constant association together would seem to point to the latter 

 conclusion. 



We have taken another interesting species (PI. 45, fig. 10) on Boris tuber culata, adhering 

 to the branchiae. It is not, however, confined to these organs, but ranges over the whole 

 surface of the body. It is minute, measuring scarcely one sixteenth of an inch long, is almost 

 colourless, depressed, ovate, the broad end forward. The thorax is composed of four segments, 

 the first being very large ; the other three narrow. The abdomen is small, and formed of 

 four or five segments, the first two being larger than the rest and not very distinctly separated. 

 The tail is bilobed, each lobe bearing five setae, two of which are very long. There are four 

 pairs of feet, three of which belong to the thorax : these are composed of several joints, 

 each, excepting the last, bearing a single spine ; the last joint is flattened and has on its 

 inner margin several setae, on the exterior margin three spines. The fourth pair of feet 

 are rudimentary ; they belong to the first segment of the abdomen, and are formed of one 

 joint, bearing at its extremity two setae. The antennas are long, slender, and composed of 

 several setose articulations. Immediately behind these, there are a pair of stout arms or feet, 

 formed of three joints, the last being furnished with two hooks. The eye is deep rose- 

 coloured. The ovigerous lobes are yellowish-white, of a cylindrical form with the ends 

 rounded ; the eggs are large, and not very numerous. 



Drawings of the above three forms have been submitted to Dr. Baird, who kindly in- 

 forms us that the last-mentioned species belongs to the genus Myasilus, and he has no 

 doubt that it is perfectly new and undescribed. The same naturalist also states that the 

 small form from Doris pilosa very possibly belongs to the genus Bomolachus ; but that it 

 appeared to be imperfectly developed. It is also new to him, and very different from any 

 species with which he is acquainted. 



We have seen another species of Brgasilus. This was taken on Antiopa cristata from 

 the south of England, and had deep orange-coloured ovigerous lobes. 



These ovigerous vesicles of the parasitic Entomostraca have sometimes been described 

 and figured as a part of the animals in which they were found. 



Distribution. With the imperfect knowledge of foreign species that we yet possess, it is 

 scarcely possible to arrive at any satisfactory conclusion concerning the general distribution of 

 the Nudibranchiata in the different regions of the globe. So far as the observations of 

 travellers go, they appear to be pretty generally diffused throughout all seas and in all 

 climates. The tropical forms are, as usual, larger and more brilliantly coloured than those of 



