The hermit crabs of the abyss, too, are not usually characterised by any very great development of spines. They find their protection in the shells they inhabit. Some of the deep-sea hermit crabs carry about with them on their shells a sea anemone, as we find to be frequently the case among the shallow-water species. Pagurus abyssorum, from a depth of 3,000 fathoms, is an example of this.
In cases where there is a scarcity of gasteropod shells the hermit crabs are obliged to find some other form of protection for their bodies. The ‘Blake’ found in the West Indies a hermit crab that had formed for itself a case of tightly compressed sand, and another curious form, named Xylopagurus rectus, makes its home in pieces of bamboo or in the holes in lumps of water-logged wood.
The last group of the Arthropoda we need refer to is that of the Pycnogonida, those curious creatures seemingly made up entirely of legs, and by some naturalists considered to be related to the Crustacea and by others to the scorpions and spiders.
Like the Brachiopoda the Pycnogonida are not usually found in greater depths than 500 fathoms. Out of the twenty-seven known genera, only five extend into the abyss, and not one of these can be called a true deep-sea genus.
There are three genera, Nymphon, Collosendeis, and Phoxichilidium, that show a very wide distribution over the floor of the ocean, and are capable of existing at the greatest depths, and of these the species of the genus Nymphon have a truly remarkable range extending from the shore to a depth of 2,225 fathoms.
‘As a rule,’ says Hoek, ‘the deep-sea species are slender, the legs very long and brittle, and the surface of the body smooth.’ They have further, either no eyes at all or rudimentary eyes without pigment, and in many cases—as, for example, Collosendeis—they are distinguished for reaching to a gigantic size compared with their shallow-water relatives.
The Tunicata is the group of animals that includes all those curious vegetable-like organisms found upon our coasts that are familiarly known as sea-squirts, or Ascidians, besides the salps, pyrosomas, and the microscopic appendicularias of the pelagic plankton.
Fig. 19.—Collosendeis arcuatus, from a depth of 1,500 metres. (After Filhol.)
Notwithstanding the apparent simplicity of their adult structure, naturalists are now agreed that they must be removed from the Mollusca, with which they have hitherto been most frequently associated, and placed in the group of the Vertebrata. It is the study of embryology that has led to this unexpected conclusion, for we find, when we study the larval forms, that they possess both a notochord and gill-slits, two features that are characteristic of the group of the Vertebrata.