456 THE ANIMAL KINGDOM. 



median foot, as seen in Clio, structures are present which might well represent the 

 lobes and valve of the funnel in a Cephalopod. But the last mentioned are points 

 on which there is great difference of opinion. See Grobben, * Morphologische 

 Studien iiber den Harn und Geschlechtsapparat, &c., der Cephalopoden ' Arb. 

 Zool. Inst. Wien, v. 1884, pp. 44-70, for a recent discussion of them and of the 

 affinities of Cephalopoda. 



BRANCH I. GLOSSOPHORA. 



Mollusca in which the head region is more or less developed, and 

 is always provided with an odontophoral apparatus and radula. 



CLASS CEPHALOPODA. 



Bilaterally symmetrical Glossophora, with the head surrounded by a 

 portion of the foot which is produced into tentacle-bearing lobes or sticker- 

 (sometimes hook-) bearing arms; with a part of the foot modified to form 

 a siphon; and a muscular contractile mantle-fold subserving both respiration 

 and locomotion. The shell is variable in structure, sometimes internal or 

 absent; when external iisually chambered. The visceral dome is elongated. 

 The ctenidia, which are usually two in number, the anus and nephridial 

 apertures, are posterior. The vascular system is highly developed, and 

 consists of a ventricle and usually two auricles, with a pair of branchial 

 hearts. The pericardium is large, the nephridia saccular. The sexes are 

 separate, and the sperm is enveloped in a spermatophore. Chromatophores 

 are remarkably well developed in the integument. 



The fore-foot has grown round the head, and its margins are produced 

 into lobes or arms ; of the arms there are eight in Octopoda, ten in Decapoda, 

 arranged in pairs. The four pairs of arms in- Octopoda are similar, and 

 are sometimes connected laterally to one another by a thin membrane 

 which may extend almost to their tips. The fourth pair (reckoning the 

 median dorsal pair as the first) is modified in the Decapoda into a pair 

 of long extensile arms which can be partially or completely (e. g. Sepia) 

 retracted into sacs at their bases. The arms are provided on their adoral 

 aspect, or at their expanded extremities in the case of the long arms of 

 Decapoda, with suckers or acetabula arranged in 1-4 rows, or with hooks. 

 The suckers of the Decapoda are stalked, and the cup has a marginal horny 

 ring armed in some instances with pointed teeth. They are sessile and 

 devoid of a ring in Octopoda. In Nautilus (Tetrabranchiata), the fore-foot 

 is divisible into an outer and inner portion. The outer portion surrounds 

 the whole head, and is thickened dorsally where it abuts against the 

 coil of the shell and forms the hood \ It carries nineteen tentacles on 



1 The fossils known as Aptychus, Andptychus, Synaptychus, and Trigonellites , which are some- 

 times found within the aperture of the shell in certain Ammonoidea, or just outside it, or entirely 

 dissociated from it, have at various times received different explanations. Of these the two which 

 are probable are the following: (i) that they were secreted by the surface of a hood corresponding 



