Comparison of Annelida with Other Turbellaria Groups
Comparision of Annelida with Nemerteans and Turbellaria Groups
The Chaetognatha and the higher Turbellaria approach the Annelida proper very closely, though from different points of view, and may be regarded as intermediate between them and the Nematodes, Tematodes, and Cestodes. The NEMERTEANS (the highest group of the Turbellaria) especially come near the annelida, notwithstanding the condition of the nervous system. They have cilia externally, and a cutis which secretes similar hyaline tubes to those of many Annelida. The muscular system is greatly developed, and in definite layers, and many swim as freely as the most active leeches. The digestive system intimately agrees, having cilia on the inner surface of the canal, a muscular oesophageal region, and a sacculated intestine, while the glandular and other elements in the wall are very similar; it differs in not passing through a buccal nervous collar, although it lies beneath the nervous system in this region. The circulatory apparatus is fairly developed, and in some corpusculated. The special plexus, in certain forms, in the oesophageal part of the digestive tract points out not only the true function of the vessels (which have been considered a water-vascular system without reference to the cephalic sacs and their ciliated vessels in the Enopla), but shows a close analogy with the anterior plexuses of many Annelida proper and Balanoglossus, and even foreshadows the branchial system of certain vertebrates, as seen, for instance, in the young Petromyzon. The appending of the branchial system to the anterior end of the digestive is characteristic. Though it is quite inaccurate to say that the Nemerteans have corpisculated fluid in the general cavity of their bodies, yet a highly organized corpusculated fluid exists in their muscular proboscidian chamber, and evidently performs important functions in their economy. Moreover, the remarkable proboscis and its sheath pass through a ring of nervous tissue consisting of the superior and inferior commissures and their connections with the ganglia. The cephalic ganglia are large, and lie over the digestive system, but the nerve-cords are separated throughout. The Nemerteans are chiefly dioecious, and the products of their sexual organs, which are developed in the form of a series of sacs on each side between the muscular wall of the body and the digestive canal, find exit by lateral pores, the contents of the male organs being often vented in clouds, as in Hermella. The young sometimes undergo a metamorphosis, e.g., the Pylidium-development of certain Anopla. To this the Tornaria-condition of Balanoglossus and the actinotrochastate of certain Gephyrea show similar features; and the three forms lead by separate channels to the Annelids proper. There is little analogy with the Tunicates, but the similarity of the development of certain Echinoderms to the three forms just mentioned is eminently suggestive.
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Annelida - Table of Contents