200 



ESTOMOSTHACA.- 



-CRUSTACEA.- 



-PiiYLLorous. 



The Sph^romians {Splmroma) live ou rocks on 

 the sea-coast, and have the faculty of rolliug them- 

 selves up in the form of a ball, when touched or 

 alarmed. They are all of small size, and several 

 species are found in Great Britain. See fig. I'JO — 

 tSji/ucromn scrndum. 



The third group are termed the Sedentary Isopods. 

 In this group the si.\th pair of false abdominal feet 

 are altogether wanting, and their mouth is formed 

 more for suction than for mastication ; as the jaws, 

 wliich all the other Malacostraca possess, in these are 

 not in existence. They are completely parasitical. 



The Bopyri, or Tall-less Crab-lice, live fixed under 

 the vault of the branchial cavity of prawns, shrimps, 

 &c., where they may be detected forming a small 

 tnmour. The males are five or six times smaller than 

 the females, and are found attached to their ahdomeu. 



The figure (lig. lUl) is that of another parasitical 

 lis. 101. 



3Iud Shrimp louse — lone tlior^cicus— a Male; h Fcm.ile. 

 species, lone thoracicus, which infests the shrimp. 



Sub-class II.— ENTOMOSTRACA. 



The Eniomostraca are for the most part extremely 

 small crustaceans, but are very numerous. Their 

 external envelope or carapace, which is of a horny or 

 coriaceous texture, is formed of one or two pieces which 

 either completely or in great part cover the body of the 

 animal. In some it approaches in appearance so nearly 

 to the form of a bivalve shell, that a person who did 

 not examine with a microscope the animal contained 

 within, would not hesitate at first to call it so. These 

 creatures are carnivorous, and are very useful in clear- 

 ing stagnant waters of ]>nlrid animal matter. Their gills 

 are attached cither to the feet or organs of mastication. 



They are preyed upon hy la:-ger animals, and form 

 the food of some of our most esteemed fishes. Some 

 of them are parasitic, living fixed upon the bodies 

 of fishes and other animals that live in the water. 

 Many undergo a series of changes, amounting to a 

 s)jecies of metamorphosis, in their progress from youth 

 to maturity. They are numerous in fresh water, and 

 many are marine, those inhabiting the ocean assisting 

 materially in producing the luniiuousness of "the world 

 of waters." By naturalists they have generally been 

 divided into three sections— BkanciuuPODA, Loi'UY- 



ROPODA, and PCECILOPODA. 



Section I.— BKANCHIOPODA (BRANcriioroDs). 



The animals belonging to this section of Entomostraca 

 Iiave the mouth furnished with jaws fitted for masticat- 



Fig. 192. 



ChirocepliaUls [Uaplianus. 



ing tlieir food. Their branchi;e or gills are many and 

 attached to the feet, which vary in number, sometimes 

 being manj' and at others few. They are in general 

 not adapted for locomotion. The antenna; are two, or 

 four-jointed and ciliated, in some serving as organs of 



motion. Tliey swim freely, and may bo observed to 

 have their brancliial feet in constant motion in the 



water, their action 

 being seldom inter- 

 ■ ~ ^ rupted, thus venti- 



lating the stagnant 

 water in wdiich they 

 for the most part 

 live, and preventing 

 it becoming soon 

 putrid. 



They are all ar- 

 I'anged in two orders, 

 PilVLLOPODA and 



Cladocera. 



OiuiER I.— PHYLLOrODS {rhyllopoda). 



In the animals belonging to this order the feet are 

 fuliaceous in structure, branchiferous or gill-bearing, 



