GENERAL CHARACTERS OF THE SUBKINGDOMS. 109 



horseshoe. Moreover, the mouth opens not directly into the oesophagus, 

 but first into a long and sometimes very wide chamber or pharynx ; and 

 sometimes other so-called atrial or respiratory chambers are met with. In 

 the Brachiopoda and Polyzoa the alimentary canal presents a neural flexure ; 

 in the Ascidioida, a haemal flexure. 



The classes of the Molluscoid subkingdom, all of which are aquatic, and 

 mostly marine, consist of the Ascidioida, which include the tunicated marine 

 animals, the Brachiopoda, and the Polyzoa or coralline polyps. It is in the 

 Brachiopoda that the alimentary canal is sometimes destitute of any outlet, 

 and that the remarkable contractile sacs, named pseudo-hearts, exist. 



Annulosa. This subkingdom is distinguished, as its name implies, by its 

 component animals having a segmented body, i. e. , a body composed of a series 

 of more or less distinctly marked annular segments or rings, named somites, 

 joined one behind the other. The body, therefore, of an Annulose animal is 

 usually elongated, or belongs to the longitudinal type. Moreover, the surface 

 of these segments is always firm, and sometimes even horny or calcareous, so 

 as to form a dermal skeleton. In the higher classes, articulated or jointed 

 limbs, also composed of external hardened integument with the soft parts 

 within, and arranged symmetrically in lateral pairs, are met with. The cen- 

 tral parts of the nervous system consist of numerous pairs of ganglia arranged 

 one behind the other in a longitudinal direction, and connected by longi- 

 tudinal cords. Usually each pair of ganglia is connected by a transverse 

 commissural cord ; but sometimes they are fused into a single mass, in which 

 case, the double longitudinal cords are likewise blended. The first pair of 

 ganglia, named supra-oesophageal or cerebral, are placed above the gullet, and 

 the cords which pass downwards and backwards to the second pair of ganglia, 

 named sub-oesophageal, embrace the sides of the gullet, which, therefore, as in 

 the Mollusca, is completely surrounded by nervous matter, and is accordingly 

 said to pass through or perforate the anterior part of the nervous apparatus : 

 the remaining ganglia, with their longitudinal connecting cord or cords, are 

 placed entirely below the alimentary canal, i. e., along the underside of the 

 body. In the higher forms, the sensory organs are highly developed, and a 

 distinct contractile dorsal vessel, segmented, and provided with valvular open- 

 ings leading into the perivisceral cavity, takes the place of the well-defined heart 

 found in the Mollusca and Vertebrata. The blood is sometimes colorless and 

 sometimes colored, and contains a few corpuscles. 



As shown in the scheme of the subkingdoms already given, the Annulosa 

 may be regarded as standing below the Vertebrata, side by side with the Mol- 

 lusca, though, in some respects, the Mollusca present the higher organization. 

 As the Mollusca have been divided into a larger and higher group, viz., the 

 Odontophora, and a smaller and lower one, consisting only of the Lamelli- 

 branchiata, so also the Annulosa may be similarly subdivided. Thus the 

 Insecta, Myriopoda, Arachnida, and Crustacea, form a large group, named 

 Arthropoda ; sometimes, even, raised into a distinct subkingdom under that 

 name, or under the title Articulata : they are characterized by always having 

 articulated limbs (dpOpor, arthron, a joint, TTO/'Y-, pous, a foot). The smaller 

 group consists of the class Annelida only, which have a softer integument, no 

 jointed limbs, but simple lateral and symmetrical appendages, such as cirrhi 

 or setae. The rest of their organization, like that of the Lamellibranchiata, 

 in comparison with the Odontophora, also presents a different and lower type 

 of organization than the Arthropoda ; and, indeed, they have by some been 

 separated from the latter to form a distinct subkingdom under the name 

 Vermes. They are, however, naturally associated with the higher Annulosa, 

 from the annulated structure of their bodies, and the double gang;lionated 

 cords of their nervous system. The Arthropoda are further distinguished by 

 the perfect bilateral symmetry, not only of the body and the organs generally, 

 but even of their digestive and reproductive systems ; also by the complex 

 structure of the head, and by their highly developed organs of vision : the 

 head has been shown to be composed of from four to six modified annul! or 

 somites (Huxley) ; the jaws move transversely, and feelers or antenna.' always 

 exist ; lastly, no vibratile cilia have been seen either in the embryonic or adult 

 condition of any of the Arthropoda. On the other hand, the Annelida, in- 



