SOME OTHER CRUSTACEA. 221 



shore crab, or occasionally on the swimming-crabs. If you 

 keep hermit-crabs even for a short period in a crowded 

 collecting-bottle, they very speedily show their discomfort 

 by quitting their shells. As they trail the lank abdomen 

 behind them, you will notice in one or two cases a large 

 rounded cylindrical body of yellowish colour attached to its 

 under surface. This is the parasite, and its only distinct 

 structural peculiarity is the reproductive orifice at the broader 

 end of the cylinder. Dissect a dead hermit, and you will 

 find ramifying through its abdomen a system of fine roots, 

 by means of which the parasite feeds itself. It has no 

 mouth, no alimentary canal, no appendages, and is chiefly a 

 mere sac of eggs. Much less conspicuous is the rounded 

 Sacculina on the abdomen of crabs, for it is partly concealed 

 by the inturned tail of the crab. Its structure is similar, 

 except that the reproductive orifice is in the middle instead 

 of at one end. 



One other small group may be considered here in con- 

 nection with the Crustacea, for though its littoral members 

 are few in number, they are very 

 common, and the group itself is one 

 of great interest. This is the Pycno- 

 gonida, or sea-spiders, including small 

 long-legged, spidery creatures com- 

 mon under stones on the shore. Two 

 are very common; one (Pycnogonum 

 littorale, see Fig. 65) occurs under 

 slightly muddy stones, and is a dirty FIG. 65. Sea-spider (Pycno. 

 yellowish flattened creature with four 9 num morale ^ 



pairs of stout knobbed legs, and a massive trunk prolonged 

 forwards into a large cone-shaped proboscis, and bearing 

 four brownish eyes on its dorsal surface. It is the most 

 sluggish and leisurely of creatures, moving, when it does 

 move, by slowly lifting one after the other its eight clawed 

 legs. The other common form is more attractive both 

 in tint and in shape. It is bright pink in colour, with 

 long slender legs about three times the length of the 

 body, and ending in long claws. In addition to the eight 

 legs which it possesses in common with the preceding, it 

 has a pair of short chelate appendages about the mouth, 

 while the male, as in the preceding form, has two very 



