68 THE VOYAGE OP H.M.S. CHALLENGES. 



the posterior gill being generally simplified, but formed, however, on the same plan as 

 in Pneumonoderma. 



Notobranchsea has also lost the lateral gill ; the posterior crest of the terminal 

 gill has also disappeared, and there only remains a gill formed of three radiating 

 crests. 



Lastly, Clione has lost every vestige of gill, as in Halo-psyche. 



A form closely resembling some very specialised Dexiobranchsea? with atrophied 

 median buccal appendage, and with the posterior ciliated ring converted into a 

 respiratory ring, might give rise : — 



1. To Sjwngiobranchsea, by the lateral gill becoming rudimentary. 



2. To Pneumonoderma, by specialisation of the posterior gill, and the development 

 of radiating crests on the annular crest. 2 



Pneumonoderma, by the disappearance of the lateral gill and buccal appendages, 

 and by the atrophy of the posterior lobe of the foot, gave rise to Clionopsis. 



A form allied to Pneumonoderma, in which the lateral gill has disappeared as well as 

 the quadrangular crest of the posterior gill, and in which the latter bears only three 

 radiating crests, might lead to Notobranchsea. 



This last genus certainly represents the forms from which Clione has been derived, 

 by the total disappearance of the branchial apparatus. 



1 The evolution of the buccal appendages in Dexiobranchsea illustrates the evolution of the genus. The first 

 specialisation was the production of suckers on the proboscis (ventrally and on the two lateral sides) ; this state is 

 shown in Dexiobranchsea simplex. The ventral suckers then united their peduncles into a single median appendage, as 

 seen in Dexiobranchsea paucidens. The number of the lateral suckers then increased, and they were arranged in two 

 rows, as in Dexiobranchsea rainuta, and Dexiobranchsea polycotyla. Lastly, the ventral appendage became atrophied, 

 and, on the other hand, the lateral suckers became more developed, and they were borne on projecting crests of the 

 proboscis, the first indication of the lateral appendages of the following genera, Spongiobranchsea and Pneumonoderma. 

 This last state is shown in Dexiobranchsea ciliata. Therefore the living species of Dexiobranchsea must be phylo- 

 genetically arranged as follows : — 



1. Dexiobranchsea simplex. 



2. Dexiobranchsea paucidens 



3. Dexiobranchsea minuta. 



4. Dexiobranchsea polycotyla. 



5. Dexiobranchsea ciliata. 



- In the Pneumonodermata, the evolution is chiefly shown in the branchial apparatus. In the most primitive 

 species the lateral gill is still long as in some Dexiobranchse, and the fringes on both lateral and posterior gill are 

 hardly developed. Pneumonoderma souleyeti corresponds to this stage. The branchial fringes then became more 

 developed, but the lateral gill continued long — Pneumonoderma pacificum. The lateral gill became shorter, and the 

 fringes more distinct, but not yet subdivided — Pneumonoderma boasi. The fringes became subdivided — Pneumono- 

 derma violaceum. Then, by specialisation of the buccal appendages the other species arose — by increase of the number 

 of suckers, Pneumonoderma peroni ; by decrease, Pneumonoderma mediterraneum. The living species of Pneumono- 

 derma must therefore be phylogenetically arranged as follows : — 



1. Pneumonoderma souleyeti. 



2. Pneumonoderma pacificum. 



3. Pneumonoderma boasi. 



4. Pneumonoderma violaceum. 



5. Pneumonoderma peroni. 6. Pneumonoderma mediterraneum. 



