N. ROSÉN, STUDIES ON THE PLECTOGXATHS. 23 



about 18 mm. belonging to Spheroides testudineus I ha ve had 

 an oppurtunity to collect. Though the spines are already 

 well developed in them, they nevertheless support the opinion 

 I have asserted about the development of the integumental 

 ossifications of the previously described Plectognaths, that 

 these ossifications have a double origin: partly from the ex- 

 teriör layer of corium, partly from the interiör one. The 

 young specimens show the two layers of the corium distinct. 

 The spines have already get a definite form. The greater 

 portion is composed of a homogeneous substance. Cells occnr 

 along the surface of the spine, and it is without doubt by 

 their action, that the homogeneous portion is formed. A fib- 

 rillated portion can be distinguished but it is very shght 

 forming only a very thin stratum between the roots. By 

 comparing this stage of the spine with the fully developed 

 organ it is evident that the growth takes place in two modes: 

 the fibrillated part increases by the addition of new bundles 

 of fibres, the homogeneous part grows by cell-action. When 

 the spine has reached the length, that it touches the epider- 

 mis, the growth of the homogeneous part must be limited to 

 the sides of the spine s. str. and the upper surfaces of the 

 roots. In young specimens the skin forms small processes 

 for each spine. In an adult stage these are not so distinct. 

 Subcutaneous tissue. Inside of the corium all round the 

 body a layer of subcutaneous tissue is developed, especially 

 well on the ventral side where the air-sac has united to the 

 body wall, as in Spheroides testudineus. That this stratum of 

 a loose highly extensible tissue is one of the most important 

 adaptions for the inflation, is evident. If the skin was not 

 loosely attached to the body by the subcutaneous tissue, the 

 great extension which is necessary for the inflation would 

 not be possible. The subcutaneous tissue shows no peculia- 

 rities as to the structure. For its relation to the air-sac I 

 refer to my paper on this question in the second section of 

 this series of studies (Fig. 13). 



Muscles. As already mentioned in dealing with the func- 

 tion of the air-sac, interesting muscles run between the corium 

 and the subcutaneous tissue. The exteriör has a träns verse 

 direction forming a circular layer round the body. The in- 

 teriör, which is longitudinal in direction, is only developed on 

 the bellv. It is by the action of these two muscles that the 



