276 BERGH,, ON URTICATING FILAMENTS IN THE MOLLUSCA. 



hepatic lobe. The sac opens to the exterior by a minute 

 pore situated at the summit. Its walls are muscular^ a cir- 

 cular layer of fibres being the most considerable element. 

 The interior is filled with urticating cells, together with 

 cysts full of closely packed filaments and free filaments. 

 Besides these are found cellaeform bodies which represent 

 probably the first stages of development of the urticating 

 filaments. The course of this development, however, is at 

 present it must be confessed but imperfectly known. But it 

 may not perhaps be incorrect to regard the battery as a 

 follicle whose secreting cells develope urticating filaments in 

 their interior. 



The genus PleurophyUidmni, according to Dr. Bergh''s 

 observation, is the only mollusc besides the Eolidpe in which 

 cnida are met Avith, and it is to be remarked that in their 

 anatomical conformation these animals appear to approach 

 very closely to the Eolidee. The minute organs described in 

 them by Dr. Bergh present, it is true, an appearance widely 

 diff^erent from that of the ordinary urticating filaments, but 

 it is by no means improbable, nevertheless, that they may be 

 morphologically homologous with them. They are minute, 

 riband-shaped bodies, with irregularis^ sinuated borders, 

 larger at one end than at the other, and probaljly possessing 

 a contractile faculty. These little bodies are enclosed in 

 sacs, within which tbey are disposed sometimes in a regular 

 radiating mass, sometimes confusedly heaped together. The 

 sacs communicate with the exterior by a minute pore. 



The use of these cnida is still problematic. Their presence 

 in the Eolidte and in Pleurophyllidium, and their absence in 

 the Dorididse, Tritoniad?e, and Phyllidife, in all of Avhich the 

 integument is much hardened by calcareous salts, might 

 lead to the suspicion that they may be defensive organs, 

 which are not required by the Gasteropods with tougher 

 hides. But one cannot conceive that fishes and Crustacea 

 would be arrested by such organs when attempting to make 

 the molluscs their prey. Dr. Bergh regards it as far more 

 probable that they serve to paralyse the minute animals 

 upon which Eolids * live — a function that is possible but not 

 yet demonstrated. 



Mr. Lewes has adduced reasons of great weight in oppo- 

 sition to the notion that the precisely similar organs in the 

 Actinia ever have this property. It is certain that the 

 urticating function implied in the name by which these 



* It should be remembered also that most species of UoUs live not upon 

 minute animals, but upon Actiniae, and perhaps other soft creatures of consi- 

 derable size. 



