ATLANT. DEEP-SEA EXPED. 1910. VOL. IIll. 



PTEROPODA. 



PeracHs Forbes. 



This genus is distinguished from the genus Limacina 

 by a whole series of characters, such as the subcircular 

 operculum, the prolongations of the head into a 

 proboscis, the symmetrically developed tentacles, 

 the broad and plate-like wings not divided into distinct 

 lateral and median lobes, and the existence of three 

 distinct visceral ganglia. 



To these characters may also be added one 1 ) already 

 mentioned by Pelseneer, namely the existence within the 

 genus Petaclis of a real ctenidium, and finally a very 

 interesting character of the radula, viz.: the existence 

 in each transverse row of a rudimentary lateral 

 tooth outside the one present in all Thecosomata. The 

 formula of the radula in Peraclis may be expressed as 



(1) 1-1-1 (!)• 



The importance of the two latter characters of the 

 genus is obvious, since they form a connection between the 

 typical thecosomatous pteropods and their tectibranchiate 

 ancestors. 



The existence of a ctenidium in the Thecosomata 

 had been denied until it was discovered by Pelseneer 

 (1906) in Peraclis triacantha. As a result of my in- 

 vestigations of the "Michael Sars" material I can not 

 only confirm the correctness of Pelseneer's discovery 

 but also supplement it in so far as I have seen similar 

 ctenidia in two other species viz.: P. reticulata and 

 P. diversa. 



In no other group of pteropoda has the existence of 

 a real ctenidium been proved, while rudimentary 

 respiratory organs are found also in the genus 

 Limacina as well as in certain species of the Cavoliniidae. 

 It is therefore of interest to note that within the genus 

 Peraclis the ctenidium varies in shape and development 

 from the feather-like organ of P. triacantha (textfig. 1 A) 

 to the twisted membrane of P. diversa (1 B) or the 

 triangular scarcely folded lobe of P. reticulata (1 C, C 1 ). 

 This varying appearance gives the impression of an organ 

 in process of reduction, which well accords with its 

 disappearance within the other genera. 



As already mentioned the radula in the genus 

 Peraclis shows the interesting archaic occurrence of a 

 rudimentary tooth on both sides of each transverse 

 row (Textfig. 2 — 4, r. t.). This rudimentary tooth has 

 been figured by Pelseneer (1906, pi. II, fig. 44 al.) in 

 P. triacantha, but in his description of the radula of this 

 species it is not mentioned at all, while in the explanation 

 of the figures it is called "lame accessoire". 



I was myself at first doubtful as to the interpretation 

 of these small additions to the common radula-type of 



') The rightsided position of the palliat caoity may well be considered a generic character of Peraclis; but, as will be shown 

 below, this is not one by which it is distinguished from other genera. 



thecosomatous pteropods, and not till I had treated a 

 great number of radulae with caustic potash did I venture 

 to believe that they represented rudimentary teeth. After 

 having prepared in the same manner the radulae of 

 very many thecosomatous as well as of gymnosomatous 

 pteropods, I can add that similar structures are found in 

 Procytnbulia, and as scarcely visible rudiments also in 

 Limacina hellcoldes, but in no other species investigated. 

 Their reactions with caustic potash are exactly like those 

 of the radula-teeth, while all other organs and tissues 

 gradually disappear during a prolonged boiling with this 

 fluid. I therefore consider myself absolutely justified in 

 looking upon these structures as rudimentary teeth, 

 probably inherited from ancestors with a greater number 

 of teeth in each transverse row of the radula. 



For a distinction between different species of the 

 genus Peraclis the m an tie- m argin is, like the shell 

 and the radula, of considerable value. In all the species 

 investigated by me a triangular balancer is found on 

 the right side of this margin (b. textfig. 1. A — C), but 

 besides this character, common to all of them, each 

 species has in the formation of its mantle-margin also 

 certain specific peculiarities. 



Peraclis reticulata d'Orbigny. 



Atlanta reticulata d'Orbigny, 1836 (p. 178, pi. 12, fig. 32— 35, 39). 

 Spinalis clathrata Eydoux and Souleyet, 1840, 1852 (p. 220. 



pi. 13, fig. 17—19). 

 Peracle physoides Forbes, 1844 (p. 186). 

 Spinalis recurvirostra Costa, 1865 (p. 125). 



The original descriptions of the species cited above 

 were all based upon the shape and structure of the shells, 

 which are characterised and figured as having a smooth 

 surface covered by a finely reticulated epidermis, a 

 character easily distinguishing this from the following 

 species, in which the surface of the shell, also covered 

 by a reticalated epidermis, is not "smooth", being in a 

 very characteristic manner folded or wrinkled along the 

 whole suture. 



When therefore Meisenheimer (1905) mentions that 

 in specimens of P. reticulata taken by the "Valdivia" 

 Expedition he has found the same ridges radiating from 

 the suture that Pelseneer described for his P. blsplnosa, 

 I should think it more probable that the "Valdivia" 

 specimens do not really belong to P. reticulata, but 

 rather to the following species P. diversa. The material 

 in my hands includes nearly 700 individuals of P. reticulata 

 and among these I have not seen one with ridges along 

 the suture. 



