EXPLANATION OF THE PLATES. 283 



PLATE VI. ALTERNATION IN MOLLUSCA AND ARTICULATA. 



Figs. 1-4. (from Allman) illustrate the embryogeny of the 

 Polyzoa (p.p. 88, 160.) 



Fig. 1. Ovum of Alcyonella fungosa, containing an embryo in 

 an early stage, with the rudiment of a single polype ; a, ciliated 

 coat of the embryo ; 6, protrusible non-ciliated portion ; c, nas- 

 cent polype. 



Fig. 2. The same, farther advanced with the rudiments of 

 two polypes c. d. 



Fig 3. The same, still farther advanced, with both polypes 

 fully formed. 



Fig. 4. A polypidom of Lophopus crystallinus, containing 

 wo polypes. This is altered from Professor Allman's figure, so 

 far as to show one of the polypes exserted. 



Figs. 5 and 6 (from Sars) illustrate the alternation of the 

 Salpw (pp. 90, 140). 



Fig. 5. A "solitary" Salpa, possessing no organs of sex, but 

 budding off from a sort of internal stolon, connected with the 

 nucleus a, a pile of sexual zooids 6. 



Fig. 6. Pile of zooids more enlarged ; a, the first formed por- 

 tion of the pile, composed of well developed zooids, and now 

 ready to break off as a Salpa-chain ; b, middle portion of less 

 advanced zooids ; c, last formed portion, in which the segments 

 have not yet acquired proper organization ; d, point of origin 

 from the nucleus. 



Fig. 7. An Ovum (of Eolis ?), which, after undergoing the 

 usual segmentation, has divided into four lobes, as if for the 

 development of as many distinct embryos (p. 160 n.) 



Figs. 8-10 (from Nordmann) illustrate the metamorphosis of 

 the Lernean Crustacea (p. 172). 



Fig. 8. Larva of Achtheres, a parasite on the Perch. 



Figs. 9 and 10. The fully-developed male and female of the 

 same the latter acquiring a much greater proportionate size. 

 Here the abdominal region, a, developed in the course of the me- 

 tamorphosis, may be considered as a gamomorphic structure for 

 the evolution of the organs of reproduction. In the female, 



