OVA OP MOLLUSCA. 540 



some Annelids, and lastly among the Mollusca, the 

 Eolidae in particular. 



Max Schultze has divided these organs into two cate- 

 gories ; one including those of a rod-like form, or the 

 bacillar, which are found pretty generally in the Turbel- 

 lariae ; and the other containing the urticating capsules 

 armed with a long filament. But the researches of other 

 observers, including Dr. Eergh, have shown that this dis- 

 tinction is unimportant. 



Dr. Bergh has devoted much attention to the urticating 

 filaments ov* cnida of the Mollusca, which are far less well 

 known than those of the Caelenterata. The existence of 

 sacs with cnida — that is to say, the existence of true 

 urticating batteries — is then at the present day a well- 

 established fact as regards the typical forms of the Eolidae, 

 i.e. in the genera Eolidida, Montagna, Facelina. 



In every case the urticating batteries are planted at the 

 extremities of the papillae . above the hepatic lobe. The 

 sac opens to the exterior by a minute pore situated at the 

 summit. Its walls are muscular, a circular 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. The genus Pleuro- 

 phyllidium, according to Dr. Bergh, is the only mollusc 

 besides the Eolidae in which cnida are met with, and it is 

 to be remarked that in their anatomical conformation these 

 animals appear to approach very closely to the Eolidae. 



The use of these cnida is still involved in doubt. Mr. 

 Lewes, however, has shown that they do not serve to 

 paralyse the animals upon which Actiniae feed. 



Ova of Mollusca. — It is interesting to watch the develop- 

 ment of the spawn of the Mollusca under a low magnifying 

 power. The ova of the Limnceus is usually found adhering 

 to the surfaces of stones, pieces of weed, or other matters in 

 the water ; they are always contained in a long ribbon- 

 like delicate ova-sac of a curious and beautiful form. 

 The mass of eggs deposited by the Doris resembles a frill 

 of lace of great beauty. In Aplysia the spawn resem- 

 ble* long strings of vermicelli, of varying tints through- 



(1) On Urticating Filaments m tlie Mollusca, by Dr. Bergh. Journ. Micros. Sei. 

 VOL ii. p. 274. 1862. 



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