LOLIGO PEALII LESUEUR 235 



lateral or lower projections mark the pallial cartilages. The 

 mantle is attached to the body in the median line on the back. 

 There are other connections, such as ligaments and muscles, 

 which, however, will be considered later. 



Integument. The integument of the squid consists of an 

 epidermis and a dermis. The former is a simple layer of epithe- 

 lial cells, which secretes a thin cuticle and covers the entire body 

 including both the inner and outer surfaces of the mantle. The 

 epidermis, which lines the funnel, forms two pairs of glands of 

 unknown function. These glands are usually know r n under the 

 name of the organ of Verrill. The pen, which is an internal shell, 

 is also a product of the epidermis inasmuch as it is secreted by 

 the shell gland. Originally nothing but an invagination of the 

 epidermis on the dorsal surface of the embryo, the shell gland 

 becomes later a closed sac inside of which is inclosed the pen. 

 In the adult the pen is a thin and narrow, transparent, chitinous 

 shell extending from the upper projection of the collar to the 

 rear end of the mantle and situated under the latter above the 

 visceral mass. The median ridge of the pen represents the line in 

 which the two free edges of the early shell gland have grown 

 together to form the sac. 



The dermis is composed of four layers: outer fibrous layer, 

 chromatophore layer, deep fibrous layer, and iridiocyst layer. 

 The changes in the color of the live squid are due to the second 

 layer and are increased by the reflection of the light from the 

 iridiocysts. Each chromatophore is composed of a pigment cell 

 and of from ten to thirty muscle cells which are attached to the 

 membrane of the pigment cell and receive a supply of nerve 

 fibres from the mantle nerves. A contraction of the muscle cells 

 results in an expansion of the pigment cell. On the under surface 

 of the fins and in all parts where the integument is not exposed to 

 light, both the chromatophores and iridiocysts are lacking. 



Skeleton. Besides the pen which serves for the attach- 

 ment of the mantle the squid possesses several cartilages. These 

 cartilages are usually called the endoskeleton, although they do 



