PRINCIPAL SYSTEMS OF ZOOLOGY 249 



tic organs are little developed; glands, especially, are rare, and mostly 

 replaced by simple tubes. 



III. The Massive Type. We may thus call the type of Mollusks, 

 for neither length nor surface prevails in them, but the whole body 

 and its separate parts are formed rather in round masses which may 

 be either hollow or solid. As the chief contrast of their structure is 

 not between the opposite ends of the body, nor between the centre 

 and periphery, there is almost throughout this type an absence of 

 symmetry. Generally the discharging pole is to the right of the recep- 

 tive one. The discharging pole, however, is either near the receptive 

 one or removed from it and approximated to the posterior extremity 

 of the body. As the tract of the digestive apparatus is always deter- 

 mined by these two poles, it is more or less arched; in its simplest 

 form it is only a single arch, as in Plumatella. When that canal is 

 long, it is curled up in a spiral in the centre, and the spiral probably 

 has its definite laws. For instance, the anterior part of the alimentary 

 canal appears to be always placed under the posterior. The principal 

 currents of blood are also in arches which do not coincide with the 

 medial line of the body. The nervous system consists of diffused gan- 

 glia united by threads, the larger ones being around the oesophagus. 

 The nervous system and the organs of sense appear late; the motions 

 are slow and powerless. 



IV. The Vertebrate Type. This is, as it were, composed of the 

 preceding types, as we distinguish an animal and a vegetative system 

 of the body, which, though influencing one another in their develop- 

 ment, have singly a peculiar typical organization. In the animal sys- 

 tem the articulation reminds us of the second type, and the discharg- 

 ing and receiving organs are also placed at opposite ends. There is, 

 however, a marked difference between the Articulates and the Verte- 

 brates, for the animal system of the Vertebrates is not only doubled 

 along the two sides, but at the same time upwards and downwards, 

 in such a way that the two lateral walls which unite below circum- 

 scribe the vegetative system, while the two tending upward surround 

 a central organ of the animal life, the brain and spinal marrow, 

 which is wanting in Invertebrates. The solid frame represents this 

 type most completely, as from its medial axis, the backbone, there 

 arise upward arches which close in an upper crest and downward 

 arches which unite, more or less, in a lower crest. Corresponding to 



