I40 CRUSTACEA PERACARIDA chap. 



harmonise so well with their surroundings that it requires an 

 experienced eye to detect them. The body is elongated and thin, 

 resembling that of a stick-insect. The first two thoracic segments 

 are more or less completely fused with the head ; the second 

 and third thoracic limbs end in claws ; the two following thoracic 

 limbs are normal in the gemis Fruto, ludimentary in Protella,iiw\ 

 absent in the remaining genera, thougli their gills remain as con- 

 spicuous flabellate structures. Tlie three hind legs are normal, 

 and the abdomen is reduced to a tiny wart at the liind end 

 of the greatly elongated thorax. 



P. Mayer has described cases of external hermaphroditism as 

 being fairly common in certain species, e.f/. Caprella acutifron><, 

 and this is interesting if we take into consideration the frequent 

 partial hermaphroditism exhibited l)y tlie gonad of Orchestia at 

 certain times of year (see p. 104). 



Fam. 2. Cyamidae. — These are closely related to the Caprel- 

 Udae in the form of the limbs and tlie reduced state of the abdo- 

 men. Cyamus ceti, which lives ectoparasitically on the skin of 

 whales, has the body expanded laterally instead of Ijeing elongated, 

 as in the Caprellids. 



Sub-Order 3. Hyperina. 



These are an equally distinct and curious group of Amphipods, 



characterised hy the large size of the head and the transparency 



,„^=^^=r— __ of the Ijody. Instead of haunting the 



littoral zone they are pelagic in habit, 



, ^ and many of them live inside trans- 



N parent pelagic INIolluscs, Timicates, or 



Jellyfish. A w"ell known form is 



/ Phronima sedentaria, which inhabits 



the glassy barrel- like cases of the 



., n- „, . , . . Tunicate Pyrosoma in the Mediter- 



i'K.. y/.- rltroiuina sedentana, "^ 



?, ilia/ Vo«oma colon V, X 1. ranean. The femaJe is often taken 



itfo^toi'i.*)'°'''^''''^''''''''' "^ t^»e plankton together with her 

 brood in one of these curious glass 

 houses ; the zooids of the Pyrosoma colony are completely eaten 

 away and the external surface of the case, instead of being rough 

 with the tentacles of the zooids, is worn to a smooth, glass-like 

 surface. It lias been observed that tlie female actively navigates 

 her house upon the surface of the sea ; slie clings on with her 



