1893.] Joint Formation Among the Invertebrata. 93 
and acting physiologically as a tubular ligamentum teres. On 
examining the different joints, we will find that commencing 
at a fixed point, as at the base of the thorax, the movable 
ring of the first abdominal somite is pulled into the fixed part. 
Then the first abdominal somite becomes the fixed point for 
the movable ring posterior to it, and so on, so that we find 
that the rings proceed away from the thorax, each is pulled 
into the opening of the one in advance. This is true of all 
those forms where the abdomen is well formed, strong and an 
active organ in the economy of the animal; when this organ, 
the abdomen, ceases to be an active organ of motion ‘as in the 
burrowing forms, as Callianassa, Gebia, some of the Squillide, 
etc., or where it is folded upon the sternum of the thoracic 
region, the muscles becoming weaker through disuse, the 
rings are not subject to the powerful muscular strain, and they 
as a rule overlap but little if at all, but lie so that the edge of 
one ring rests upon the edge of another. In those forms 
where degeneration of the abdomen has proceeded so far as 
not to have even the usual deposit of calcareous matter, as in 
the hermit crabs, there are simply indications of rings on the 
abdomen, and this organ is but little more than a fleshy sae 
containing some of the viscera, and supplied with a few mus- 
cles which act together, with the form of the organ, to keep 
the abdomen curled so that it may hold as a hook, the animal 
within the molluscan shell which it habitually occupies. 
In forms as highly developed as the cray-fish and the lob- 
ster where the calcareous deposits are great, there have crept 
in many modifications. One of the main features in the 
articulations, of the rings of the abdomen is the “lock” or 
hinge which allows no lateral movement in the tail whatever; 
the lateral movement-is also prevented by the overlap of the 
pleura. This lock consists of a rounded prominence on each 
of the rings projecting forward from a posterior segment into a 
socket in the anterior one. The movement in any two seg- 
ments so formed takes place in the plane, the axis of which 
passes through these two points so that the are described by 
the movement is only in a vertical plane. The terga of the 
segment is much arched in the lobster, and the upward 
motion is limited after a certain point is reached by the dome 
