2i6 ANNULATA. 



accordance therewith, it is lined with a chitinous cuticle. 

 In addition the cuticle in the stomach has a number of 

 hard sclerites which form the gastric mill. This apparatus 

 has a median tooth and two lateral teeth worked by power- 

 ful muscles. Further, the aperture between the cardiac 

 and pyloric portions is guarded by strainers, or small pro- 

 cesses, covered with "hairs." Digestion of the food is 

 apparently confined to the region of the mesenteron. 



The sclerites of the lobster are moved by a complex series 

 of muscles lying inside the body or limbs. There are two 

 __ series of muscles — ( i ) the flexors which by con- 



traction bend the abdomen or the limb ; (2) the 

 extensors which straighten it. In the limbs, at least, these 

 are attached to the arthrodial membranes by tendons, but in 

 some cases to the edge of the sclerite. A cross-section of 

 the abdomen shows the powerful flexors, the contractions of 

 which bend the tail and drive the lobster backwards through 

 the water, and above them the much thinner extensors. 



The anterior flexors are attached in the thorax to the 

 endophragmal skeleton, which consists of parts of the ecto- 

 derm with cuticle ; these have grown in from the ventral 

 surface during development. Hence the endophragmal 

 skeleton does not constitute an endoskeleton. 



The lobster can swim gently forwards by the action of 

 the swimmerets, it can creep in any direction by its legs and 

 it can shoot rapidly backwards by contraction of the tail. 



The skeleton being an exoskeleton, it has already been 

 noticed in the external features. We need only emphasise 

 the tucking of the ectoderm into the stomach 

 and into the ventral region of the thorax, the 

 sclerites in each case forming the gastric mill and the endo- 

 phragmal skeleton. 



The vascular systems of the lobster are in a peculiar 

 condition. In the Annelida and Archicoelomata we could 

 -- distinguish two vascular systems. The larger and 



more spacious, contained within the mesoderm, 

 was called the coelom and was mainly nutritive and motor ; 

 the smaller consisted of fissures and small sinuses lying between 

 the three primary layers, was called the blood-vascular or 

 haemocoelic system, and was usually respiratory and ex- 

 cretory. 



