46 Guide to the Mollusca. 



boring animals are described in the Guide to Marine Boring 

 Ani nulls and are illustrated in a case in the Central Hall. 



The 'Watering-pot shell ' (Brechites), of the family Clava- 

 gellidae, is a long tubular structure with an expansion at one end. 

 perforated Like the ' rose 'of a watering-pot. Lt doesnot represent 

 the shell of an ordinary bivalved mollusc, and on looking carefully 

 near the perforated end (' the rose "), two small valves (the true 

 shell) will be seen embedded in the surface. They are found with 

 the ' rose ' downwards, buried in mud or sand at low water on the 

 shores of the Indian and Pacific Oceans. 



Order 4. SEPTIBRANCHIA. 

 {Case 204.) 



The Septibranchia are a small group in which the gills are 

 transformed into a muscular partition, which divides the branchial 

 chamber into cloacal and inhalant portions. The Septibranchia 

 are connected with the Eulamellibranchia through such forms as 

 Lyonsia, in which the first steps in the development of muscular 

 tissue in the gills can be seen. Respiration is effected by the 

 inner surface of the mantle, the muscular septum by its con- 

 tractions keeping up a continuous stream of water. From the 

 form of the alimentary canal and the contents of the stomach it 

 is inferred that these animals are carnivorous. 



The Septibranchia are marine and may be found at great 

 depths. Poromya, Cetoconcha, and Cuspidaria are the best known 

 genera, of which the first and last are found in British waters. 



Class V. CEPHALOPODA. 



(Cases 205 to 208.) 



The Cephalopoda include the Octopus, Cuttlefish, Squid, 

 Nautilus, &c. They are Mollusca in which the edges of the foot 

 are divided and extended to form long appendages and are in 

 addition carried forward and joined in front of the mouth, which 

 thus comes to lie in the middle of the foot. A portion of the 

 latter (the epipodium) is expanded into a muscular tube, the 

 funnel, by which the water contained in the mantle-cavity is 

 expelled. The animals, which are exclusively marine, move head 

 downwards when on the bottom. The position is changed in 

 swimming, as they move with their visceral sac in front by means 

 of jets of water vigorously expelled from the mantle-cavity 

 through the funnel. The mouth is usually furnished with a 

 formidable pair of horny beaks (mandibles), and the closely 

 concentrated nervous ganglia are protected by a cartilaginous 

 sheath. In some members of this class the eye possesses, in 

 addition to a cornea, retina, and lens, a number of accessory 



