Another bivalve so unlike a crustacean, among which it has been placed, I may venture to describe among Lamp-shells. I refer to the barnacle (Lepas) generally met with covering the bottoms of ships. These, as in the former genus, are more interesting to the microscopist in the early stage of existence, and also for the curious parasites known to infest them. The barnacle protrudes through its two valves six pairs of slender, bristly, two-branched filamentous limbs, which keep up a constant sweeping motion, and whereby it secures its supply of food ([Fig. 363]). When first hatched the young are in the Nauplius stage, being furnished with a median eye and three pairs of flagellated appendages. After enjoying a free life the larva moults and passes into a second stage, in which with its two eyes and compressed carapace (shown in [Fig. 364]) it so nearly resembles a Daphnia. Before these thoracic appendages entirely disappear they first change places, and then each is seen to be provided with a sucker; by this means the larva fixes itself to its permanent resting-place, while a cement gland pours out a secretion that glues it firmly to the point of attachment chosen. These Cirripedes are not true parasites, inasmuch as they do not extract nourishment from the body to which they are attached.
Fig. 363.
1. Spat of oyster, some ciliated; 2. Barnacles attached by footstalks.
One species, the Proteolepas, is in the adult stage a maggot-like, limbless, shell-less animal found living within the mantle chamber of other members of the same order, while the root-headed Cirripedes (Peltogaster curvatus, as [Fig. 364], No. 1) live parasitically upon higher crustaceans.
Echinodermata.—This sub-kingdom includes the star-fishes, stone-lilies, sea-urchins, feather-stars, and sea-cucumbers, some of which have been already alluded to, and are so well known that they need no lengthy description, while of the fossil sea-urchins of our chalk formations, the Pentremites and Crinoids, whose silicious remains are so abundant and so familiar to naturalists and geologists, but little remains to be said. They are chiefly interesting to the microscopist from their calcareous and silicious appendages, known as spicula. In the sea-urchin, brittle-star, or feather-star, the outer body surface consists almost wholly of a deposit of calcium carbonate, combined in the form of little plates built up into a rigid “test,” whereas in the star-fish it usually forms a kind of scaffolding, between the layers of which there stretches a firm leathery skin. Among the sea-cucumbers, the living specimens of which present extraordinary variations both in form and character, the deposit consists chiefly of small spicules which grate when the skin is cut with a knife. If a thin section of the skin is examined under the microscope, the spicules are seen to be profusely distributed in the middle layer. The same deposit takes place in the stalked column of a crinoid and in sea-urchins (Echinodermata), which has tended to preserve them in the fossilised state. [Fig. 365] is selected as exhibiting to perfection the Medusa-headed Pentacrinoid. This echinoderm differs in two characters: first, its microscopic structure is that of a meshwork deposited in the spaces of a network of soft tissue; secondly, that each element, whether a spicule or a plate, is, despite its trellised structure, deposited around regular lines of crystallisation (shown in [Plate IV]., Nos. 89 and 90). Owing to these characteristics the minutest portion of an echinoderm skeleton is readily recognised, even when fossilised, under the microscope. Even the species of the sea-cucumber can be determined by the shape of their spicules.
Fig. 364.—Parasitic Barnacles.
1. Peltogaster curvatus; 2. Nauplius larva of Parthenopea; × 200.
Another noticeable feature in the radiate structure is that in many cases it gives to the animal a star-shape, to which the names of star-fish and brittle-star are given (see [Plate IV]., No. 91, and [Plate XVII]., f and n). The ordinary five-rayed star-fish is found everywhere around the English coasts. This constant arrangement of organs holds good in the majority of the echinoderms; it can be detected in the Holothurians, where, beside the feathery tentacles of the head, rows of shorter sucker-like processes will be found, which in some instances extend the whole length of the body, the fixed number of rows being also five in their internal organs. Hence these animals were formerly grouped under Radiata. But if a sea-cucumber or sea-urchin be dissected, a marked distinction will be found between them, in one portion of the organism in particular: the intestine is shut off from the rest of the body-cavity, often coiling round inside. Examine a star-fish or sea-urchin on the under-surface of the rays, and, passing in five bands from top to bottom, a number of small cylindrical processes are seen gently waving about; these lie in two rows with a clear space between them, and are termed in consequence ambulacrum. They end in sucker-like discs, which enable the animal to attach itself, or pull itself against strong currents.