590 THE BIOLOGY OF MARINE ANIMALS 



the head of a beautiful worm, Nereis bilineata, which rapidly glided out round the 

 Crab's right cheek, and, passing between the upper and lower foot-jaws, seized the 

 morsel of food, and, retreating, forcibly dragged it from the Crab's very mouth. 

 I beheld this with amazement, admiring that, though the Crab sought to recover 

 his hold, he manifested not the least sign of anger at the actions of the Worm. . . . 

 I was surprised to observe what a cavern opened beneath the pointed head of the 

 Nereis when it seized the morsel, and with what force comparatively large pieces 

 were torn off and swallowed, and how firmly the throat-jaws held the piece when 

 it would not yield. Occasionally it was dragged quite away from the Crab's jaws, 

 and quickly carried into the recesses of the shell; sometimes in this case he put in 

 one of his claws and recovered his morsel; at others he gave a sudden start at 

 missing his grasp, which frightened the Worm and made it let go and retreat ; but 

 sometimes the latter made good his foray, and enjoyed his plunder in secret. 



Among Crustacea many copepods are commensal in habit : Hersiliodes, 

 for example, lives in the sand galleries of the burrowing shrimp CaUianassa. 

 Porcelain crabs of the genus Polyonyx are frequent associates of various 

 tubicolous polychaetes, for example Loimia and Chaetopterus. Apart from 

 protection certain of these commensal forms are undoubtedly dependent 

 upon water currents created by their hosts. 



Inquilinism 



Inquilinism is one route to parasitism, and in some groups, such as 

 copepods, species can be found that represent all steps from simple com- 

 mensals sheltering within the host to specialized parasites. In the copepod 

 sub-order Notodelphoida, living in ascidians, forms like Notodelphys and 

 Doropygus have biting mouth parts resembling those of free-living cope- 

 pods. They live in the branchial chamber of their hosts, and seize food 

 particles brought in by water currents. Certain other forms such as 

 Enter ocola and Enter opsis have moved into the stomach or epicardial 

 tubes of the ascidian and feed on the fluids of the host. 



Pinnotherid (pea) crabs are usually regarded as inquiline commensals, 

 but their behaviour is more suggestive of parasitism. They commonly live 

 in the mantle cavity of lamellibranchs, but occasionally in other animals 

 such as echinoids, holothurians and ascidians, or in the burrows of poly- 

 chaetes and burrowing shrimps (CaUianassa). The pea-crab obtains oxygen 

 and food from the currents of water passing through the mantle cavity of 

 the lamellibranch, and competes with the mollusc by removing some of 

 the mucus-strings passing towards its mouth. As a result of its activities 

 it often weakens or even injures the gills of its bivalve host. Usually there is 

 only one pea-crab in a mollusc, and some forms are rather specific in their 

 hosts (72, 100). Other inquiline Crustacea are the shrimp Pontonia found in 

 the mantle cavity of Pinna, and Typton living in the sponge Desmacidon. 

 There are also certain inquiline polychaetes, for example the polynoid 

 Hipponoe occurring in the mantle cavity of the goose barnacle Lepas, and 

 inquiline nemertines among the Malacobdellidae, which are provided with 

 suckers and live in the mantle cavity of lamellibranchs (49). 



Certain teleosts have also adopted the habit of living inside other 

 sedentary animals and display suitable modifications of structure. The genus 



