TYPE CRUSTACEA. 



397 



tially parasitic, still retain more or less perfectly tlie segmen- 

 tation and general appearance of free-swimming forms, the 

 modifications which they have undergone affecting principally 

 the antennae, which are modified for purposes of adhesion to 

 the host, the mandibles, which are piercing organs, and in some 

 cases the maxillae, which may, like the antennae, become hook- 

 like. Frequently, however, the body assumes aberrant forms, 

 as in Fhilichthys (Fig. 178, A), and the segmentation may en- 

 tirely disappear, as in Fenella, Lerncea, Chondracanthtcs, Achtheres 

 (Fig. 178, £), and Anchorella, these last two forms presenting 

 a peculiar modification of the second maxillae in the females, 

 the two appendages fusing at their tips to form a chitinous 

 adhesive disk which serves as an organ of adhesion. In the 

 majority of these forms, as 

 already noted, the thoracic 

 appendages may become more 

 or less rudimentary; indeed 

 even in the less modified 

 forms, such as Ergasilus, the 

 appendages of the fifth thora- 

 cic segment may be wanting. 



3. Suborder BrancMura. 



In the Branchiura the 

 cephalic and thoracic seg- 

 ments are fused together to 

 form a shield-shaped cephalo- 

 thorax, while the abdomen is 

 small and divided into two 

 platelike halves which have 

 a rich blood-supply, appar- 

 ently serving respiratory pur- 

 poses, and in the males 

 contain the testes. 



The basal joint of the an- 

 tennules (Fig. 179, at') is devel- 

 oped into a strong hooked process, and the mandibles and 

 first maxillaB, which are stylet-like, are enclosed ia a tube 



Fig. 



179. — Argulus foliaceus 

 Glaus). 

 at' = antennules. 



I = digestive gland- 

 ma; = second maxilla. 

 oc= eye. 

 t = testis. 



(after 



