PHYLUM ARTHROPODA 



579 



The Sessile Barnacles or Acorn-shells (Balanus) have no stalk 

 (Fig. 455), the head-region being short and broad. The scuta (s) 

 and terga (t) support a valvular carapace, through the opening of 

 which the feet are protruded, and the whole animal is surrounded 

 by a sort of parapet (s7.:) formed of six calcareous pieces. One 

 of these, dorsal in position, is the carina, the others appear to be 

 represented by small ossicles developed on the peduncle of certain 

 stalked forms such as Pollicipes. 



Many of the Eucirripedia are parasitic. Some of these (Petmrca, 

 &c.), parasitic in Actinozoa, resemble the attached forms in 

 essential respects ; others (Alcippe), parasitic in the shells of 

 Molluscs and Cirripedes, have abdominal but no thoracic feet. 



A 



FIG. 455. Balanus. A. external view ; B, anatomy. a\. antcunules ; ad. adductor muscle ; 

 m. muscles of scuta and terga ; o, edge of parapet ; ov. ovary ; ovi. oviduct ; s. scutum ; 

 sk. parapet; t. tergum ; wo. female aperture. (From Lang's Comparative Anatomy, after 

 Darwin.) 



Proteolepas, also parasitic on other Cirripedes, has a maggot-like, 

 segmented, limbless body, and a suctorial mouth. 



The Rhizocephala are represented bySacculina (Fig. 456), parasitic 

 on Crabs, and Pcltogaster on Hermit-Crabs. Both genera have the 

 appearance of an immense tumour (ks) on the abdomen of the host, 

 showing no sign of segmentation, no appendages, no mouth or 

 anus. From the attached end go off a number of delicate root- 

 like filaments, which extend through the body of the host and 

 absorb nutriment. Obviously degeneration is here as complete as 

 it can well be, and nothing but the developmental history of the 

 parasite (p. 599) would justify its inclusion among the Crustacea. 



The most striking general character in the external features of 

 the Malacostraca is the limitation in the number of segments. The 



