124 THE CRUSTACEA 



selves up into a ball when alarmed. A gigantic Isopod, measuring 

 9 inches in length by 4 inches in breadth, far exceeding all other 

 living Isopods in size, and called Bathynomus giganteus, was dredged 

 up from a depth of 955 fathoms on the north-east of the bank of 

 Yucatan, and north of Tortugas, by Prof. Alexander Agassiz. The 

 Gribble (Limnoria terebrans), so destructive to all woodwork below 

 tide-mark, is also an Isopod. 



The Branchiopoda, or gill-footed Crustacea, are all aquatic, 

 the greater number having a shell composed of two parts or valves, 

 in which they are more or less completely enclosed, or in the 

 form of a buckler, which envelops a large part of the animal. The 

 little Daphnia pulex, the so-called Water-flea, is a Branchiopod 

 belonging to the order Cladocera. 1 



The Lophyropoda, or stiff hair-footed Crustacea, includes the 

 orders Ostracoda (Shell-covered) and Copepoda (Oar-footed), 

 familiar members of which are the fresh-water Cypris, the fresh- 

 water Cyclops, both inhabitants of ponds ; the marine Cetochilus 

 or " Whale-food," which, although quite minute in size, yet swarms 

 in such vast numbers at certain seasons of the year as to colour 

 the sea for a mile or more in extent ; and the parasitic Copepod, 

 Argulus foliaceus, which is found upon various fresh- water fishes. 



The Cirripedia 2 includes the interesting Acorn and Stalked 

 Barnacles (the Balanida and Lepadidce), which pass through a 

 most extraordinary metamorphosis. " Almost everyone/' writes 

 Darwin, " who has walked over a rocky shore knows that the Bar- 

 nacle, or ' acorn-shell,' is an irregular cone, formed generally of 

 six compartments, with an orifice at the top, closed by a neatly- 

 fitted, movable lid, or operculum. Within this shell the animal's 

 body is lodged, and through a slit in the lid it has the power of 

 protruding six pairs of articulated cirri, or legs, and of securing 

 by their means any prey brought by the waters within their reach. 

 The basis is firmly cemented to the surface of attachment. The 

 whole shell, basis, and operculum consists of the first three seg- 

 ments of the head, modified into a singularly constructed carapace, 

 which encloses the mouth and the rest of the body. The anterior 

 extremity of the shell is situated in the centre of the basis, where, 



1 Greek, klados, a branch, and her as, a horn branching-horn, in allusion to their 

 antennae. 



2 Latin, cirrus, a curl, and pes, a foot curl-footed. 



