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 Protel/la, 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, eg. 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 
Rey ee eae Tunicate Pyrosoma ne the Mediter- 
9, ina Pyrosoma colony, x 1. ranean. The female is often taken 
SOE ea Gerstaecker i the 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 has been observed that the female actively navigates 
her house upon the surface of the sea; she clings on with her 
