464 THE ANIMAL KINGDOM. 



It is then said to be hectocotylised. In Argonauta Argo, Ocythoe tuberculata 

 (= Parasira catenulatd), Tremoctopus violaceus, and T. quoyanus, this arm 

 or Hectocotylus is set free, and reproduced after each act of sexual congress. 

 It developes within a sac, which then bursts, disclosing a large arm with 

 peculiar suckers, and a terminal saccule. The latter bursts in its turn, 

 setting free a 'NeedhamV filament. The modified arm is in all cases 

 charged with spermatophores, which appear to gain access to it by an 

 aperture on its aboral surface. When it is detached it is carried away 

 by the female in the branchial cavity 1 . The ova are either inclosed, each 

 in a special capsule, which is attached to some foreign object ; or carried in 

 the shell (Argonauta] ; or a number are imbedded in a mass of gelatinous 

 material, fixed (Loligo) or floating. Segmentation is partial, the protoplasm 

 collecting in a terminal disc. The yolk is not inclosed in the alimentary 

 canal, but is lodged in what appears to be a median part of the foot, in the 

 head, neck, and mantle. Cilia have been observed on the blastoderm. 



The Cephalopoda are marine, some littoral like Octopus, others pelagic 

 like Spirula, Sepia, &c. All are carnivorous, and some, e.g. Architeuthis, 

 attain a gigantic size. The oldest Cephalopoda belong to the Nautiloidea, 

 and appear in Cambrian strata. Of existing genera, Loligo is found in the 

 Lias, Ommastrephes and Sepia in the Oxford Clay. 



The class Cephalopoda is divisible into two orders. 



I. Tetrabranchiata. Shell external ; straight or coiled ; chambered ; the last 

 chamber inhabited by the animal, the remainder filled with gas. A membranous 

 tube or siphuncle traverses the chambers. Mostly extinct : one genus only, Nautilus, 

 living, with the following characters. The part of the foot surrounding the mouth 

 forms lobes which bear sheathed tentacles ; the sipho is formed by two free folds. 

 The eyes are simple open sacs ; there is a pair of osphradial papillae. The viscero- 

 pericardial sac opens externally by two apertures. There are two pairs of gills and 

 nephridial sacs ; two genital ducts, the left rudimentary in both sexes. No auricles, 

 no branchial hearts nor ink-bag. 



(1) Nautiloidea. Shell straight or bent ; coiled, and then snail-like, or disc-like; 

 aperture simple, sometimes much contracted; ventral side indicated by a notch. 

 Sutures mostly simple, or waved; rarely denticulated. Septa concave forwards. 

 Sipho often contracted by internal deposits ; siphuncular collars generally directed 

 backwards. Protoconch conical, with a cicatrix. 



(2) Ammonoidea. Shell usually coiled and disc-like ; rarely snail-like or straight; 

 aperture simple, or with lateral and ventral processes. Sutures waved, and often 

 denticulated. Sipho always marginal ; never contracted by internal deposits. Proto- 

 conch globular or ovate. Aptychus or Anaptychus generally present (p. 456, note). 



The form of the protoconch, mode of commencement of the sipho, absence of 



1 The ventral lobe of the buccal membrane in the female Sepia contains two pouches ; in Loligo 



>ne^ After congress spermatophores are found on the buccal membrane, and after a time spermatozoa 



i the pouches in question, which they reach by their own movements. The female Loligo has been 



observed holding the newly laid ova in front of the mouth. Viailleton, C. R. 101. 1885. See ante, 



