Crustaceans. 51 



The true barnacles are occasionally attached to other 

 animals, without deriving from their hosts any greater benefit 

 than that of support. Many Crustaceans, however, are, strictly 

 speaking, parasites, living entirely at the expense of other 

 aquatic animals, often of their own class. These parasitic 

 forms are for the most part extremely degenerate, and, as in 

 the barnacles, the clue to their sj'stematic position is 

 afforded by their larval stages. A good example is offered 

 by a near relative of the barnacles [Sacculina carcini) appearing 

 as an oval, cushion-like body attached to the under-surface 

 of the abdomen of many individuals of the shore crab, and also 

 of other crabs. The parasite shows no indication of its crusta- 

 cean nature, all locomotory and masticatory organs are 

 absent, food being drawn from the host by means of long 

 branched filaments which ramify throughout the bod}' of the 

 crab. We know that the Sacculine is a Crustacean, and must 

 be placed among the Cirripedes, because its early life-history 

 is almost identical wdth that of a barnacle, the development 

 including the Nauplius and C3'pris stages, after which the larva 

 introduces itself into the body of its future host (Fig. 59). 

 Similar degeneration in connexion with parasitic habits 

 occurs in many of the other divisions of the Crustaceans. 

 Among the Isopods, species of Bopyrus are common parasites 

 living under the carapace of shrimps and prawns, causing 

 the peculiar swellings so often noticeable on the sides of 

 these animals (Fig. 60). 



The Arachnids are nearly related to the Crustaceans. 

 In addition to microscopic Acarians (Mites), they are represented 

 on our rocky shores by two or three species of Pseudo-scorpions 

 of the genus Obisium, tiny creatures about a line in length, 

 half-way between Spiders and Scorpions, living among stones 

 overgrown with fucus, which keep them damp when the tide 

 withdraws. The original figure of Obisium maritimum (Fig. 61) is 

 reproduced here. The Myriopods are represented by two species 

 of Chilopod Millipedes : Geofhilus maritunus (Fig. 62), first 

 discovered at Polperro in Cornwall and since found in Devonshire, 

 Brittany, Normandy and Scandinavia ; and G. submarimis , 

 originally described from near St. Malo and since obtained on 

 Jersey and at Polperro. They live in fissures in rocks and under 

 stones and seaweeds washed by the sea at high tide. These are 

 yellowish-brown in colour and measure a little over an inch in 

 length. 



Even the class of insects, or Hexapods (six-legged), must 

 be mentioned before dismissing the Arthropods, for, many 



