336 KOREN AND DANIELSSEN ON THE 



their appearance as two pyriform organs, composed of rounded 

 cells (figs. 8 & 9h) ; their lower extremity is the thicker, and their 

 centre is filled by a mass of deeply coloured pigment-granules. 

 The keel- shaped heart appears at the same time, as well as the 

 mouth and the rudiments of the proboscis. Sars, Loven, Nord- 

 mann and Vogt have been unable to discover any heart in the 

 first periods of development, and we have ourselves sought it 

 in vain in many Nudibranchiate genera ; we are therefore led 

 to conclude that the first period of development passes by 

 without the existence of this organ. It is not the same with 

 the Pectinibranchiata, at least with Buccinum undatum and 

 Purpura lapillus. In these the heart has already appeared in 

 the interval between the twenty-third and the twenty-eighth 

 day. Grant* was the first to observe the heart in Buccinum 

 undatum, and to draw attention to its strong pulsations. To 

 the same observer we owe the discovery of the two rounded organs 

 provided with vibratile cirrhi upon the sides of the head, by 

 the aid of which the embryos move about in Purpura, Trochus, 

 Nerita, Doris, and Eolis. Subsequently, Lund, Sars, Loven, 

 Nordmann and other naturalists, have confirmed the exact- 

 ness of these observations. The heart lies upon the dorsal 

 side, a little to the left, is a little twisted and completely un- 

 covered at its first appearance ; for different organs which cover 

 it afterwards, such as the roof of the branchial cavity, are still 

 rudimentary. In the place where the heart is about to appear, 

 a grayish, transparent, finely granular mass, having a rounded 

 form, arises ; it is attached to the lobes and the foot, and pre- 

 sents no sensible movement. Soon, slight contractions are per- 

 ceptible ; the form becomes more and more marked, and at last 

 it resembles a great clear vesicle. Two or three muscular tubes, 

 of extreme delicacy, are now traceable upon its parietes and 

 effect its contractions. In this primitive state, the heart possesses 

 the form which it will subsequently retain (fig. 6e). In propor- 

 tion as its parietes increase in thickness and magnitude, the 

 muscular tubes are multiplied, trans verse tubes appearing and be- 

 coming filled with a transparent and colourless liquid (figs. 7? 8 

 & 9 e) ; we have often counted the pulsations, and have found 

 that their rate varies ; on the average, there were some forty to 

 * Edinburgh Philosophical Journal, 1827, vol.vii. p. 121. 



