THE EVIDENCE FROM DEVELOPMENT. 



225 



members are illustrated by the Dentalium or " toothshell," other- 

 wise often named the "elephant's tusk-shell" (Fig. 144, B), from 

 its obvious resemblance to the latter structure. In the early 

 history of Dentalium, we find obvious resemblances to the de- 

 velopment of the bivalves. First, segmentation or division of the 

 egg takes place (Fig. 145, A). Next, the young "toothshell" on 

 escaping from the egg appears as a rounded body, and possesses tufts 

 of cilia for swimming, and likewise has a "flagellum " (z) in front (B). 

 The body then lengthens and develops seven circlets of cilia (Fig. 145, 

 C),the resemblance between the young "toothshell" in this guise and 

 an embryo worm (Fig. 157, B) being unmistakable. Then the shell is 

 formed by the " mantle " (Fig. 145, D, a) as before, and the cilia form 

 a " velum " (z) at the upper extremity of the body, the young condition 

 of the bivalve being closely imitated at this stage. The shell, at first 

 open below, unites by its lower edges to form the toothlike structure 

 of the perfect animal, and with the further growth of the internal 

 organs (E), Dentalium becomes the mature animal. There cannot 

 exist a doubt that, as the lowest gasteropod, and as a poor relation 

 of the higher whelks and snails, Dentalium's life-history shows, as 

 might be expected, the closest approach, firstly, to animals of lower 

 grade than mollusca, and, secondly, at a more advanced stage 

 that of the " veliger" (Fig. 145, C, D) to the bivalves themselves. 



Equally interesting is the 

 chronicle of development which 

 those little limpet-like animals, 

 the Chitons (Fig. 139), present 

 to our view. These latter forms 

 are found adhering to the rocks 

 and stones at low water, like 

 the neighbouring limpets. They 

 agree with the limpets in being 

 Gasteropods; but their structure 

 is, if anything, lower than that 

 of the familiar molluscs just 

 mentioned), and their shell is 

 not univalve, but composed of 

 no fewer than eight pieces (Fig. 

 1 39, A), arranged one after the other, on the animal's back. No definite 

 head, however, is found in the chitons, this lack of front extremity being, 

 as before, a proof of lowness and democracy in the scale of Gasteropod 

 society. The general aspect of a chiton is unquestionably more like 

 that of an "articulated" or jointed animal than of a mollusc, in 

 which latter we do not expect to see segments of any kind represented. 

 It is likewise a fact of much interest that these chitons are a remark- 

 ably ancient group of the Gasteropod-class. They may, it is true, be 



Q 



FIG. 145. DEVELOPMENT OF DENTALIUM. 



