Habits.—The newly-captured specimens of both P. caudatus and H. spinulosus are of a flesh colour, with a somewhat metallic sheen. According to Apel, the latter lived in an aquarium for more than five months, whilst the former died during the first month. When first introduced into the aquarium they immediately began to busy themselves in the mud or sand at its bottom, and very seldom showed themselves above it. They forced their way into the sand by alternately contracting and extending their introvert, and the Priapulus arranged itself so that a portion, often a very small one, of its caudal appendage was exposed to the water; this fact supports the view that the appendage is respiratory in function. When the animal buries itself deeply, the appendage does not relinquish its position at the surface of the sand, but stretches itself until it in some cases surpasses the length of the body. On the other hand, Halicryptus (Fig. 219), according to the same observer, lies with the anterior end, the mouth, projecting from the surface of the sand, or else it curves itself, so that both ends project into the water.
Fig. 219.—Halicryptus spinulosus v. Sieb. × 6. a, Dark line indicating the position of the ventral nerve-cord; d, mouth surrounded by spines.
Leckenby, who described specimens of P. caudatus which were found by fishermen searching for worms for bait in the outer harbour at Scarborough at half tide, states that they live in sandy clay in U-shaped tubes, at a depth of about 9 inches, the tubes opening at each end on to the surface of the sand. The fishermen of this district call them "sea mushrooms."
Halicryptus casts its cuticle in May and September; it becomes loose first at the hinder end, and the split between it and the skin grows forward until the animal lies free in a cuticular mantle. After some days this is split, and the animal frees itself from it; the cast-off cuticle includes for a short distance the lining of the mouth, the anus, and the two generative pores.
III. Order Echiuroidea.
Anatomy.—The most striking peculiarity of the Echiuroidea, as opposed to the other two families of the Gephyrea, is the presence of a solid dorsal outgrowth of a portion of the head, forming the proboscis. The nature of this proboscis is something quite different from that of the introvert of the Sipunculoidea; it would appear to correspond to an extension, in the members of the last-named Order, of that part of the head which is dorsal to the mouth and is covered by a peculiar pigment-epithelium, often in continuity with the brain. In its outgrowth this portion of the body has carried with it the nerve-ring and the vascular ring, which both surround the mouth. The proboscis is found in all the genera with the exception of the aberrant genus Saccosoma.
Fig. 220.—A, Bonellia viridis Rol., ♀; B, B. fuliginosa. Both nat. size. a, Grooved proboscis; b, mouth; c, ventral hooks; d, anus.
The body of the female Bonellia viridis, one of the best known species of Echiurids, is shaped like a small sausage, and is usually about 2 inches long. The proboscis arises from the anterior end, and is extremely extensible. At the distal end the proboscis splits into two short arms, which are often recurved; along the whole ventral surface runs a groove lined with cilia, which by the approximation of its edges can be converted into a tube. At the bottom of the proboscis the groove opens into the mouth. Echiurus; Thalassema, and the female Hamingia have short proboscides, which do not bifurcate but otherwise resemble those of the female Bonellia.