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. DIVISION 3. HOPLOCARIDA. The carapace leaves at least four of the thoracic somites distinct. The eyes are pedunculate. The mandibles are without a lacinia mobilis; there are no oostegites, the eggs being carried in a chamber formed by the maxillipedes. The hepatic caeca are much ramified, the heart is greatly elongated, stretching through thorax and abdomen, with a pair of ostia in each segment. The spermatozoa are spherical, and there is a complicated and peculiar metamorphosis. =Order. Stomatopoda.= [Illustration: FIG. 98.—Lateral view of _Squilla_ sp., × 1. _A.1_, _A.2_, 1st and 2nd antennae; _Ab.1_, 1st abdominal segment; _Ab.6_, 6th abdominal appendage; _C_, cephalothorax, consisting of the head fused with the first five thoracic segments; _E_, eye; _M_, 2nd maxillipede; _T_, telson. (After Gerstaecker and Ortmann.) ] The Stomatopoda are rather large animals, occasionally reaching a foot in length, all of which exhibit a very similar structure; _Squilla mantis_ and _S. desmaresti_ are found on the south coast of England not very frequently; but they are very common in the Mediterranean, living in holes or in the sand within the littoral zone of shallow water. They differ from all the other Malacostraca by a combination of characters, and Calman proposes the term HOPLOCARIDA for a division equivalent to the Peracarida, Eucarida, etc.