140 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 genus Proto, rudimentary in Protella, and 

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

 spicuous flabellate structures. The three hind legs are normal, 

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

 of the greatly elongated thorax. 



P. Mayer has described cases of external hermaphroditism as 

 being fairly common in certain species, e.g. Caprella acutifrons, 

 and this is interesting if we take into consideration the frequent 

 partial hermaphroditism exhibited by the gonad of Orchestia at 

 certain times of year (see p. 104). 



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

 lidae in the form of the limbs and the reduced state of the abdo- 

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

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

 as in the Caprellids. 



Sub-Order 3. Hyperina. 



These are an equally distinct and curious group of Amphipods, 

 characterised by the large size of the head and the transparency 



of the body. Instead of haunting the 

 littoral zone they are pelagic in habit, 

 and many of them live inside trans- 

 parent pelagic Molluscs, Tunicates, or 

 Jellyfish. A well known form is 

 Phronima sedentaria, which inhabits 

 the glassy barrel -like cases of the 

 Tunicate Pi/rosoma in the Mediter- 



iu,. vi. Phroiiima sedentana, " 



9, in z Pyrosoma colony , x i. ranean. The leniale is often taken 



(After Clans, from Gerstaecker j ' th plankton together with her 



and Ortmann. ) 



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 has been observed that the female actively navigates 

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



