194 



HARDWICKE'S SCIENCE-GOSSIP. 



without an exception, and the definition I have just 

 given only applies to water mites in their nymph 

 and perfect states, for, as will appear hereafter, the 

 larval mite has but six legs. These creatures are 

 again divided into two great sections — the swimming 

 mites (i.e. mites having special bristles attached to 

 some of their legs for the purpose of swimming) and 

 the crawling mud or marsh mites ; these latter cannot 

 swim, but walk about under the water on the mud 

 and weeds, they have not the special bristles attached 

 to their legs, although these are by no means destitute 

 of hair. The water mites are again very conveniently 

 divided into those possessing two eyes, Hygrobatides, 

 and those having four eyes, Hydrachnides. The 

 former were supposed to be river mites, or mites 

 living in moving water ; and the latter, pond mites, 

 or those to be found in stagnant water ; this is how- 

 ever, not correct, for all the kinds may be found in 

 stagnant ponds, as well as in slowly moving water, 

 such as fish-ponds and rivers'; so these divisions, 

 though artificial, are so clear that the merest tyro 

 cannot well make a mistake, and it is a great point 

 to be able to clear away a wliole division by the pre- 

 sence or absence of a single character. We now 

 come to the separate families, and here we shall 

 meet with many difficulties made more complex by 

 the fact that different authorities use the same names 

 for very different families. I shall, however, follow 

 Koch as nearly as I can, believing that all his remarks 

 and figures are the result of actual observation, and 

 not copies from other authors ; and, for teaching, I 

 prefer a poor figure from nature to a more elaborate 

 one concocted from copies, because these latter, 

 when the object itself has not been studied, are as 

 likely to make prominent any slight error, or acci- 

 dental defect, as the most important parts of the 

 figure. Before we proceed further, perhaps a table 

 showing how far we have got, may be of service : 



(i. Swimming J "j-^r^^''^?''. ''"*' apparently 



Watermites. mites 1 „''""T ^j"""''' l^f- 



I (Hydrachnides, with four eyes. 



(2. Mud or marsh mites, all with four eyes. 

 {To be continued.) 



NOTES ON SOME COMMON SEA SLUGS. 

 By Dr. P. Quin Keegan. 



{Continued from page 182.] 



7^ ORIS yOIINSTONI is another species very 

 J-^ like the preceding, but it is not so large, and 

 it is not common anywhere. It is of a lightish tint of 

 yellow, speckled with a few small brown spots ; the 

 pimples that stud the cloak are more minute and of 

 equal size ; the branchial plumes are fifteen in number, 

 tripinnate like the last, and when unfolded wear the 

 likeness of a kingly crown in miniature. This species 

 measures about if inch long at most. Its tongue 

 bristles with an armature of stoutish hook-hke teeth, 

 which are marshalled in twenty- four double ranks as 



before, each rank comprising about twenty-five teeth, 

 the total number being therefore about 1200 (five on 

 the extreme left and right of each row are slenderer 

 than the other twenty, and are set at a different 

 angle). 



If during the early spring-time we take a walk 

 among the loose tide-forsaken boulders which strew 

 the beach near low-water mark, we shall probably 

 observe high up upon a wave-worn stone a cluster 

 of soft, flattish, flabby, warty bodies clinging together 

 as if for mutual support and protection, or in the 

 spirit of good-fellowship. These are specimens of 

 Doris hilaincllata, a species that may be considered 

 as the commonest and most numerous of all the 

 British sea-slugs. 



Its cloak is of a yellowish-grey colour, but it is 

 flatter, rougher, and more dappled and blotched with 

 brown than that of either of the foregoing species. 

 The branchial plumes when expanded from their com- 

 mon enclosure are seen arranged in a horse-shoe forn» 

 to the number of from twelve to twenty-nine ; they art- 

 very fresh -looking, and so developed that ample 

 aeration is provided for the ovarian and hepatic blood 

 of this animal, and probably such is required on 

 account of its fecundity and the large size of its egg- 

 frills. The cloak is charged with spicula which are 

 stoutish, blunt at the points, and bent at the centre, 

 where there is usually a tiny spur or branchlet. There 

 is a very remarkable difference in the jaw and palate 

 of this species, as compared with those of the foregoing. 

 There is a spiny collar here fitted for grasping, anil 

 the tongue, instead of being square, is narrow and 

 strap-shaped, and armed with twenty-eight rows of 

 only two teeth on each side (or 112 in all), the inner 

 onesof which'are very large and of a triangular jjat- 

 tern, slightly curved above, while the outer ones are 

 rudimentary and blunt ; and in the centre of the organ 

 there is seen an ear-shaped membranous fold. 



If during the autumn we turn over the tresses of 

 seaweed that flow so luxuriantly over the crests of the 

 huge stones near low-water mark, we shall possibly 

 discover a small convex, egg-shaped organism of a 

 light waxy-blue colour, of a transparent substance, 

 and studded with rather long webs of flabby flesh, 

 while from one extremity there issues a slender fringe 

 of minute filaments. This is Doris filosa, a species 

 that seems to prefer the close covert of sheltering 

 seaweed to the flaring exposure of the bare rock. 

 The gills, which are seven to nine in number united 

 at the base, are contractile only, not retractile into 

 any cavity, hence, when we place the animals in sea- 

 water, these organs forthwith unfold and assume their 

 shape which is starlike, and moreover they have white 

 midribs and a double row of globular hollow bodies 

 in each stem. Under a hand-lens the skin is seen 

 extensively inlaid with large spicula, which are of the 

 usual rough and clumsy pattern characteristic of the 

 tribe. As compared with the sponges, these nudi- 

 branchs are exceedingly clumsy and unmechanical 



