Sub-Class 2. Malacostraca. Order 4. Isopoda. 



215 



destitute of limbs; wlien they leave the brood-pouch they usually 

 possess the general form of the perfect animal, but they still lack 

 th.e last pair of thoracic legs. 



Some of the Isopods are marine, some fresh-water, and others 

 terrestrial (in damp places) . They are essentially adapted for walk- 

 ing or for running, but some swim by means of the abdominal 

 appendages. Many are parasitic. 



1. In tlie Nortli Sea live, for example, several species of Idothea, relatively 

 elongate animals, -with tlie last pair of abdominal appendages modified to form 

 a valve-like opercvdum, covering over the otters. One species of this genus 

 {I. tricuspidata), which lives on the shore among the sea-weed, is chai-acterised 

 by exhibiting many different colom- variations (speckled in different ways). 

 Fui'fcher, the small Gribble [Limnoria iereftratis), which gnaws holes in the 

 wood-work of harboui-s, etc., and is sometimes veiy destmctive. 



2. The flat, long-legged A sell us (Asellus aquaticus) is common in fresh, 

 water lakes amongst decaying vegetation. 



3. Many species of Oniscidse are ten'estrial (e.g., genus Oniscus). They are 

 chai'aoterised by the rudimentai-y first antenna, and by the very minute terminal 

 segment of the abdomen. In addition to the usual aiTangement of gUls, some 

 possess a kind of lung ; the outer lamina of some of the abdominal appendages 

 encloses a branching cavity, with a sUt-like aperture, which has undoubtedly a 

 respiratory function. The Oniscidse are light-avoiding animals, of insignificant 

 coloui- ; some {Armadillidiiim) can i-oU themselves up like some of the Myriapods, 

 to which they have a supei-flcial resemblance. 



4. The numerous pai'asitio Isopods live principally upon Ksh and Crustacea. 

 They offer a gi-adation in adaption to a parasitic life, similar to that in the 



Fig. 177. 1 Gepon elegans, a Bopyride from the branchial cavity of a Crab, ? (the male 

 (?_is fixed to the base of the abdomen). Dorsal view. 2 — 3 Portunion Kossmanni, g 

 {from the right) and 9 (from the left), an Entonisoide, which is parasitic in a Crab : 

 S enlarged much more than ? . 



II — VIII thoracic segments, 1 — 6 abdominal segments, 3' lateral process of the third 

 abdominal segment (these processes light, the abdominal appendages dark), C head ( + 

 first thoracic segment), Ca abdomen, H^, E^, ffj first, fifth, sixth abdominal appendages, 

 R marsupial lamellse. — After Giard and Bonnier. 



