II. Annelida Oligochaeta
General Description. The Body. Circulatory System. Digestive System.
II. The A. OLIGOCHAETA are annelids without tentacles, cirri, or specialized branchial processes. Bristles, variously grouped, from two to eight and upwards in each transverse series. They are hermaphrodite, and the young undergo no metamorphosis.
The body is enveloped in a delicate cuticle resembling that of the Polychaeta, and pierced by many pores. Underneath is a cellulo-granular hypoderm, with numerous glands. The setae are simple, bifid, or hair-like. The muscular layers are an external circular, and in internal longitudinal, marked by the bristle sacs. The foregoing tissues enclose a perivisceral space, with the usual septa and the characteristic corpuscles in a coagulable lymph, which performs important functions in the economy.
The circulatory system consists essentially of a dorsal trunk situated over the digestive chamber, carrying the blood from behind forwards, and a ventral or sub-intestinal conveying the fluid in the opposite direction. The former is broken up into a plexus of vessels in the anterior segments and generally gives off a largebranch (perivisceral) on each side in the others. The periviscerals in certain segments behind the anterior plexus are enlarged, and so evidently contractile as to have received the name of "hearts," and propel the blood into the ventral vessel. In the other segments the periviscerals pass to the latter, either with or without into branches -- in the form of cutaneous plexuses, which are greatly developed in this group. Branches pass from the ventral to the intestinal wall, inosculating either others from the dorsal, and forming a rich capillary network. In the Lumbrici there is also a third longitudinal vessel underneath the nerve-cord, and it is in communication with both dorsal and ventral, especially by the cutaneous system. The blood is generally reddish, devoid of corpuscles, and coagulable. Respiration seems to be effected by the cutaneous and special plexuses, aided, probably, in some by the currents of water in the ciliated digestive canal.
The digestive system is in the form of a simple, straight, more or less muscular tube, differentiated into osesophagus, crop, gizzard, and sacculated intestine in Lumbricus, and coated internally with the usual glandular elements (certain glands anteriorly being supposed to be salivary), and lined internally with cilia. In the same group a peculiar closed fold of the chamber constitutes the typhlosolis of authors. The anus is situated at the posterior end of the body. In the Limicolous group the structure is less complex -- pharynx, osesophagus, and intestine only being present. The nervous system consists of a pair of cephalic ganglia, from each of which a trunk passes by the side of the oesophagus to join the ventral chain, which gives off branches in each segment to the surrounding parts. In Lumbricus another small gangliated chain lies over the commencement of the alimentary tract. The nervous system agrees in intimate structure with that of the Polychaeta. Besides touch, and in some sight, the special senses are not much developed. Several have epidermal papillae connected with cutaneous nerves, which probably aid tactile sensibility.
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Annelida - Table of Contents