Sub-Order 2. Laemodipoda.
Fig. [96].—Caprella grandimana, × 4. a, Abdomen; g, gills; t, 3rd (first free) thoracic segment; t′, 8th thoracic segment. (After P. Mayer.)
Fam. 1. Caprellidae[[113]] are also chiefly littoral forms, swarming among rocks covered by algae, though they are by no means so easy to detect as the Gammaridae and Tanaidae which haunt similar situations. In a basinful of algae or Polyzoa taken from the rocks fringing the Bay of Naples, the latter are easily collected, the Tanaidae always crawling out of the weeds in the direction of the light, while the Gammarids dart about in all directions; but the Caprellidae, with their branching stick-like forms, 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 conspicuous 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 Caprellidae in the form of the limbs and the reduced state of the abdomen. 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.
Fig. [97].—Phronima sedentaria, ♀, in a Pyrosoma colony, × 1. (After Claus, from Gerstaecker and Ortmann.)
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 transparent pelagic Molluscs, Tunicates, or Jellyfish. A well known form is Phronima sedentaria, which inhabits the glassy barrel-like cases of the Tunicate Pyrosoma in the Mediterranean. The female is often taken in 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 thoracic legs inside, while the abdomen is pushed out through an opening of the Pyrosoma case behind, and by its alternate flexion and extension drives the boat forwards, the water being thus made to enter at the front aperture and supply the female and her brood with nourishment.