a shallow-water species (4–70 fathoms). C, lens; V, crystalline cone; R, rhabdom; N, nerve. (After Beddard.)
The eyes of all the deep-sea species are relatively larger than those of the shallow-water ones, except Serolis gracilis, whose eyes seem to be disappearing.
But these large eyes of the deep-sea species of Serolis are not capable of any greater perceptive power. In fact, the evidence of degeneration they show, both in minute structure and in the diminution of pigment, proves that they can be of very little use to these animals for perception.
The Globigerina ooze is perhaps the best known of all the different deep-sea deposits. It was discovered and first described by the officers of the American Coast Survey in 1853. It is found in great abundance in the Atlantic Ocean in regions shallower than 2,200 fathoms. Deeper than this, it gradually merges into the ‘Red mud.’ It is mainly composed of the shells of Foraminifera, and of these the different species of Globigerina are the most abundant. It is probably formed partly by the shells of the dead Foraminifera that actually live on the bottom of the ocean and partly by the shells of those that live near the surface or in intermediate depths and fall to the bottom when their lives are done.
So abundant are the shells of these Protozoa that nearly 95 per cent. of the Globigerina ooze is composed of carbonate of lime. The remaining five per cent. is composed of sulphate and phosphate of lime, carbonate of ammonia, the oxides of iron and manganese, and argillaceous matters. The oxides of iron and manganese are probably of meteoric origin; the argillaceous matter may be due to the trituration of lumps of pumice stone and to the deposits caused by dust storms.
M, mouth; S, ciliated groove; T, tentacles. Each tentacle is perforated by a single large aperture.
A fact of some importance that supports this hypothesis, as regards some parts of the ocean at least, is presented by the sea-anemones.
Many of the shallow-water Actinians are known to possess minute slits in the tentacles and disc, affording a free communication between the general body cavity or cœlenteron and the exterior.
In many deep-sea forms the tentacles are considerably shorter and the apertures larger than they are in shallow-water forms. It is difficult to believe that such forms, perforated by, comparatively speaking, large holes, could manage to live in rapidly flowing water, for if they did so they would soon be smothered by the fine mud that composes the floor of all the deep seas. In fact anemones of the type presented by such forms as Sicyonis crassa are only fitted for existence in sluggish or still water.