30 INVERTEBRATE ZOOLOGY. 



Order 4. Cirripedia Sessile forms , enclosed in an inverted 



position in a calcareous 3-valved shell. 

 Generally six pairs of 2-branched legs, 

 modified to form attenuated many jointed 

 cirri. Larval stage a free-swimming 

 nauplias which soon becomes fixed by 

 the first antennae. This after passing 

 through a so-called "Cypris" stage, de- 

 velops into the adult. 



Sub-order 1. Lepadoidea Forms with stalk and flexible valves — 



skeleton mainly composed of scuta, terga 

 and carina. -^ Lepas. 



Sub-order 2. Balanoidea Forms without stalk, skeleton reinforced 



by lateral pieces, which, with carina and 

 rostrum, form a calcareous tube. 

 if Balauus. 



Sub-order 3. Rhizocephala ... . Degenerate parasites upon crabs (Bra- 



chyuro). Body consists of a sack, from 

 which grow countless root-like threads 

 (=the stalks, morphologically) which 

 penetrate the flesh of the host. Recog- 

 nizable as Cirripedia only in larval life. 

 if Saccnlhia. 



Sub-class II. Malacostraka Generally large forms with a constant 



number of segments (20) consisting of a 

 head with five, a thorax with eight and 

 an abdomen of seven segments. The first 

 two portions are often fvised to form a 

 cephalo-thorax of thirteen segments. 

 Only the first six abdominal segments 

 Ijear appendages, and of these the last 

 pair is generally modified and united with 

 the terminal segment to form a caudal 

 ajjpendage. The paired reproductive ori- 

 fices of the male are found upon the last 

 thoracic segment, and those of the female 

 upon the third from the last. Develop- 

 ment sometimes direct — generally with a 

 inetamorijhosis. Nauplias larvae appear 

 only in a few primitive forms. 



