Only Carcinonemertes has been classed as a para- 

 site. It lives on the gills of various crabs when it is 

 young, and then moves to the egg masses, both feed- 

 ing on the eggs and living as a commensal by eating 

 any small animals it can find as it clings to its host. 

 Adults of Carcinonemertes carcinopliila are about 1 

 inch long and orange- or brick-red. 



Commensal nemerteans live mostly in tunicates, 

 sponges, or bivalves, sharing the food in the host's 

 feeding currents. Common in the mantle cavity of 

 various clams on European and both American 

 coasts is Malacobdella grossa, a short, white, thick 

 worm, with an adhesive disk at the rear. It creeps in 

 leechlike fashion. The genus to which it belongs con- 

 stitutes a separate order of nemerteans. 



The other three orders contain all the more typical 

 elongated worms; they are distinguished from each 

 other mostly by internal characters, such as the ar- 

 rangement of the muscle layers. The paleonemer- 

 teans, with an unarmed proboscis, include such forms 

 as Tiihiilaniis. species of which are shown in Plate 37 

 and below. Also with unarmed proboscis are the 

 heteronemerteans, among them Linens and Cere- 

 bratulus. The latter is a very large, firm, and flat- 

 tened worm which lives in burrows in sand or mud 

 and swims actively through the water. The hoplone- 

 merteans, with an armed proboscis, are divided into 

 two suborders. In one the members have at the tip 

 of the proboscis a single stylet, a straight or curved 

 thorn which pierces and holds prey. These include 

 many quite common shore forms such as Aniphi- 

 ponis; the very slender Emplectonema. found 

 among mussels and barnacles on pilings; and Para- 

 neinerles peregrina of the American west coast, of- 

 ten a rich purple on the upper surface. The parasitic 

 or commensal Carcinonemertes belongs here, as do 

 various commensal species, the fresh-water forms, 

 and also the land nemerteans. In the second subor- 

 der, members have on the tip of the proboscis not one 

 large stylet but a large number of minute ones. These 

 worms include some shore species, but most float or 

 swim in the open sea far below the surface. Many are 

 broad, flattened worms, of yellow, orange, pink, or 

 red hues. The drifting types are quite gelatinous, the 

 swimming ones equipped with tail and sometimes 

 also with side fins. 



Ttihiilaniis annulatus, a long, slender ribbon worm 

 common on European shores, may be several feet long. 

 It lives in a mucous tube under stones or in crevices 

 at low shore levels. ( Rupert Riedl and Encyclopae- 

 dia Britannica Films) 



132] 



