EXPLANATION OF THE PLATES. 2KI 



Plate YI. — Alternation in Mollusca xnd Autk-tlata. 



Figs. 1-4. (from Allman) illustrate the eiubryogcny 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, ciliat^-d 

 coat of the embryo ; h, protrusible non-ciliated portion ; f, uas- 

 cent polype. 



Fig. 2. — The same, farther advanced Avith the rudimeni^ «-i 

 two polypes c. d. 



Fig 3. — The same, still farther advanced, with botli polyjtt^ 

 fully formed. 



Fig. 4. — A polypidom of Loplwjyns crystallinuSy containing 

 wo polypes. This is altered from Professor Allman'H figure, no 

 far as to show one of the polypes exserted. 



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

 Salpce^i-p-p. 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 h. 



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

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

 ready to break oflf as a Salpa-chain ; b, middle portion of lew* 

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

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

 from tlie nucleus. 



Fig. 7. — An Ovum (of Eolis ?), which, after undergoing th«' 

 usual segmentation, has divided into four lobes, as if for th»- 

 development of as many distinct embryos — (p. ItiO n.) 



Figs. 8-10 (from Nordmann) illustrate the metamorithosih i.f 

 the Lernean Crustacea — (p. 172). 



Yig. 8. — Larva of Achtheres, a parasite on the Perch. 



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

 same — the latter acquiring a much greater proportionate hiie. 

 Here the abdominal region, a, developed in the c(»urse of the me- 

 tamorphosis, may be considered as a gamomorphic structure for 

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



