110 RELATIONS OF MAN WITH EXTERNAL NATURE. 



stead of a hard external skeleton and jointed limbs, have a soft body with 

 simple appendages ; the head never contains even four modified somites 

 (Huxley) ; the organs of sense, especially the eyes, are very simple ; there is 

 no distinct valved dorsal vessel communicating with the perivisceral cavity ; 

 most of them possess, either in the embryonic or adult condition, vibratile 

 cilia on, or in some part of, their bodies ; and, lastly, they are nearly all pro- 

 vided with peculiar vessels, named pseudo-haemal, which frequently contain a 

 colored corpusculated fluid. 



Annuloida. Standing below the Mollusca, we found simpler soft-skinned 

 animals grouped together under the name Molluscoida, as suggested by Milne- 

 Edwards ; and so below the Annulosa are arranged, by Huxley, under the 

 name Annuloida, which is intended to show their relations with the Annu- 

 losa, the class Scolecida, containing certain marine worms, the entozoa or 

 parasitic worms, and the rotiferous animalcules, and the class Echinoder- 

 niata, or star-fishes. These Annuloid animals approach the lowest Annulosa, 

 i. e., the Annelides, in the worm-like form of the bodies of many of them ; in 

 the frequent presence of cilia, at least in the embryo condition ; in the posses- 

 sion of a peculiar set of vessels, named the water-vessels, in the Scolecida, and 

 the ambulacral vessels, in the Echinodermata, which may represent the pseudo- 

 hasmal vessels of the Annelida. But the Annuloida are distinguished from the 

 Annulosa by the imperfect segmentation of the body, or by the complete absence 

 of segmentation, and by the non-existence of bilateral symmetrical limbs or 

 appendages. The nervous system never presents the double longitudinal gan- 

 glionated cord, but consists of either one, two, or four supra-oesophageal ganglia 

 situated in the fore-part of the body, above or upon the gullet, from which 

 delicate branches merely ramify forwards through the head, and backwards 

 through the body ; in the Echinodermata, in accordance with their horizon- 

 tally radiated form, the ganglia, which might be termed circa-cesophageal, are 

 proportionally multiplied, are connected with cords surrounding the oral 

 aperture, and give off radiating branches. Eye-spots are present in the Roti- 

 fera and in some Echinodermata, but the other sensory organs are rudimentary 

 or absent. In most of the Annuloida, moreover, that remarkable mode of de- 

 velopment is observed, by which the ova do not immediately form perfect 

 animals, but larvse or embryonic forms, within which, by a subsequent process 

 of evolution, perfect animals are produced. This kind of development is known 

 as alternate generation. 



Coslenterata. This extremely natural group, established as a subkingdom 

 by Frey and Leuckart, consists of animals, the bodies of which have a much 

 simpler structure than those even of the lowest Annuloid or lowest Molluscoid 

 animals ; although the radiated form, common to both the class Actinozoa or 

 sea-anemones, and the class Hydrozoa, which includes the Medusse, Acalepha 

 or sea-nettles, and Hydroid Polyps, suggests resemblances with the Echino- 

 dermata on the one hand, and with the Polyzoa on the other. The body of 

 the Coslenterata is hollow ; the alimentary canal, destitute of special glands, is 

 extremely short and simple, for it has but one external aperture, viz., the oral 

 opening or mouth, its hinder end opening widely into the cavity of the body 

 itself; hence the name Coelenterata (/co7/or, koilos, hollow ; evrEpor, enteron, in- 

 testine). The walls of the body are also characteristically simple, being com- 

 posed of an outer layer named the ectoderm, and an inner layer named the 

 endoderm ; both are composed of nucleated cells, and apparently in the sim- 

 plest forms, as in the Hydra, possess the same physiological properties, for 

 they are equally capable of digesting food received into the hollow of the body, 

 even when this is turned inside out. Around the oral opening are usually 

 found numerous prehensile tentacles, usually hollow, and never provided with 

 vibratile cilia upon the surface, like the tentacles of the Polyzoa. Most of the 

 Coelenterata have, in their ectoderm, little oval elastic sacs containing, besides 

 fluid, a long barbed and serrated filament, which is projected beyond the sac 

 on any irritation, and so acts offensively or defensively, destroying soft animal 

 prey, and even irritating the human skin. These sacs are named nemato-cysts 

 or thread-cells, and their irritating qualities have given rise to the term sea- 

 nettles, applied especially to the Acalepha. Somewhat similar bodies, it may 

 bs added, are found in certain Mollusca and Scolecida. The nervous system 



