GENERAL LAWS OF ANIMAL STRUCTURE. 277 



ing serial repetition, would come the nervous system. 

 Ideally the nervous system of articulata consists of a 

 series of ganglia, one to each somite or body segment 

 and presiding over that segment, but sympathetically 

 connected by a continuous thread with one another, and 

 all with the cephalic ganglion. But as we pass up the 

 scale, modification and consolidation, differentiation and 

 specialization, proceed pari passu, as already shown on 

 page 87. 



2. MOLLUSCA. 



Here, again, we find an entirely different general plan 

 of structure. In vertebrates we have an internal skele- 

 ton, the axis of which consists of segments modified ac- 

 cording to place and function. In articulates we have 

 an external skeleton still more obviously segmented and 

 modified. In mollusca alone, among all the great de- 

 partments, we have no segitientation, no serial repetition 

 of ideally similar parts. This want of repeated seg- 

 ments is its most distinctive peculiarity. In mollusca, 

 also, we have no true — i. e., locomotive — skeleton at all, 

 either internal or external, but only a protective shell. 



General Character. — The general characters of this 

 type are mostly negative: 1. A want of a true locomo- 

 tive skeleton. 2. A complete want of segmentation of 

 any system. 3. The presence of a soft, mucous, sensi- 

 tive surface wherever not covered with shell. 



The most important of these is the complete absence 

 of segmentation. It follows from this that there can be 

 no serial homology. There is, of course, a special ho- 

 mology — i. e., a homology of part with part in different 

 orders and classes of mollusks, but it is obscure and 

 little studied. I shall not attempt its exposition. 



Why there is no Segmentation in this Type. — It is seen 

 that homology, especially serial homology, is mostly lim- 

 ited to the skeleton and nervous system — i. e., to the or- 



