SOME OTHER CRUSTACEA. 219 



last, to form a kind of forceps. There are no true append- 

 ages on the next two segments, but merely two expanded 

 respiratory plates (rp). In the mature female these seg- 

 ments also give rise to four incurved lamellae, which together 

 form a brood pouch containing the transparent eggs. The 

 last three thoracic segments each bear a pair of well- 

 developed legs directed backwards. The abdomen (ab) is 

 reduced to a mere knob. There are several species which 

 are not very easy to distinguish from one another ; the 

 commonest is, perhaps, Caprella tuberculata. 



We have given very few examples of these sessile-eyed 

 Crustacea, because they are not of great general interest, and 

 those who are desirous of pursuing the subject further will 

 find Bate and Westwood's British Sessile-eyed Crustacea a 

 comprehensive and readily accessible work. 



The Amphipods and Isopods do not complete the Crus- 

 tacea, for there are in addition a considerable number of 

 other orders, all comprised in the Entomostraca or lower 

 Crustacea, as contrasted with Amphipods, Isopods, Schizo- 

 pods, and Decapods, which are all included in the Mala- 

 costraca or higher Crustacea. The Entomostraca are usually 

 of small size, consist of a variable number of segments, and 

 have no gizzard or gastric mill. A large number of them are 

 parasites, often very degraded parasites, and many others 

 are water fleas, such as may be found in any pool. The 

 latter form an important part of the food of fishes and other 

 marine animals, but cannot be considered in detail here. 

 Of the Entomostraca we shall consider only four common 

 examples, all belonging to the order Cirripedia. 



Our first example is, perhaps, the most abundant animal 

 of all on many shores. This is Balanus balanoides, the 

 common acorn -shell, often so abundant as to whiten the 

 shore rocks, and also covering shells, posts, and almost every 

 available surface within tide-marks. At low tide the little 

 white cones look dead and desolate enough ; but if you 

 watch a mass of them exposed to the action of the incoming 

 water, you will find the scene changed indeed. As the 

 white water breaks foaming over the rock, and trickles off 

 more slowly, you will see each tiny shell open and protrude 

 a delicate fringe, which opens and closes in frantic haste as 

 if its owner were aware that the water would soon be gone. 



