1902 Encyclopedia > Annelida > IV. Annelida Discophora.: Annelida Discophora: Description. The Body. Circulatory System. Digestive System. Nervous System.

Annelida
(Part 10)




IV. Annelida Discophora

Annelida Discophora: Description. The Body. Circulatory System. Digestive System. Nervous System.

IV. The A. DISCOPHORA (leeches) are ringed forms (fig. 17), without lateral appendages (except in Branchellion), but possessing an anterior and posterior sucker. There is no specialized respiratory system. They are generally hermaphrodite, and often ectoparasitic.


Hirudo medicinalis image

Fig. 17 -- Hirudo medicinalis, L.

Externally there is a delicate porous cuticle; beneath lies a thick glandular cutis, which pours out an abundant secretion of mucus, as in the Nemerteans. The muscular system consists of an external circular coat of considerable strength, from which various vertical or decussating bands pass at intervals, powerful longitudinal bundles being held in the intermediate spaces, especially laterally. There are also special muscles connected with the pharynx and other parts. Each end of the body is generally formed into a flattened muscular sucker, consisting apparently of several amalgamated segments. By aid of the latter organs they move from point to point, and they can also progress rapidly by swimming like the Nemerteans. The perivisceral space is quite absent. The circulatory system shows a main dorsal, a ventral, and two lateral trunks, all communicating by branches in each ring. In Branchiobdella the blood is corpusculated. The blood is probably aerated on the cutaneous, and perhaps, in some cases, on the digestive surface. The digestive system consists of a mouth opening into the anterior sucker, a muscular pharynx, oesophagus, a large stomach with various caecal diverticula, and an anus which opens in front of the posterior sucker, though sometimes into it. In some (e.g., the common leech), the mouth is furnished with three horny serrated teeth. The nervous system is composed of two cephalic ganglia, which supply branches to the eyes when present, and the usual gangliated ventral cord -- giving off branches to the surrounding parts. The gullet passes through the connecting trunks as in the former groups. The last ganglion in the ventral cord of Clepsine is larger than the others; and, as in the Polychaeta, the ganglia do not always correspond in number with the rings. Other nerves occur in several e.g., the leech, which has a trunk running along the dorsal surface of the digestive canal. Eye-specks are present in some, and touch is generally much developed. The cupuliform organs in the cutaneous tissues of the head and anterior region in certain leeches are supposed to be connected with the latter sense.





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