2. The Endoparasitic Development of Peltogaster curvatus (Kossmann). 55 



up into the compact green mass of filamentous roots which characterises the adult Peltogaster. 

 But Peltogaster agrees with Sacculina in the essential feature that at an early stage of its 

 endoparasitic development the body consists of a growing centrai tumour and 

 roots composed of embryonic actively dividing cells, without a trace of further 

 differentiation into the organs of the adult. The method of growth of the tumour 

 and roots of Peltogaster, while differing from Sacculina in its greater simplicity, is highly in- 

 structive, because from it we gain a perfect confirmation of the idea which we formed as to 

 how the young Sacculina interna passes along the intestine of its host to its definitive position. 

 We supposed in the case of Sacculina that the process of growth was brought about by the 

 centrai tumour as it increased in size continually throwing off roots towards the anterior region 

 of the crab, while the tumour itself travelled in the opposite direction, namely toward the 

 posterior part of the crab. The necessary result of this method would be that the oldest roots, 

 i. e. the first formed, would always be further away from the centrai tumour and the region 

 of differentiation of the external body than the younger roots. Now this in Sacculina is hard 

 to prove because of the roots being so highly branched and complicated, but in a young 

 Peltogaster internus it is quite easy to see that the state of things postulated by our 

 theory is exactly the truth. Because it may be held as a safe rule that the age of a root is 

 indicated by its length reckoned from the tip to the point where it joins the main stolon, 

 and in Piate 6 fìg. 1 which represents a typical Peltogaster internus it is seen that the 

 roots become regularly longer and longer the further they are situated from the point (n) where 

 the external body will be differentiated. We take this to be a certain proof of the method 

 of growth postulated for Sacculina and again indicated above, namely the change of position 

 by growth of a centrai tumour which leaves in its train a system of roots, the oldest of which 

 are always further away from that part of the centrai tumour which has advanced furthest 

 and which will give rise to the future external body of the parasite. 



The differentiation of the mantle and visceral mass takes place, as already mentioned, 

 at the anterior end of the centrai tumour (n in fìg. 1). When this differentiation is stili far 

 from complete, this portion of the body acquires its deep red colour, so that the presence of 

 a young developing Peltogaster at this stage can, under favourable circumstances, be recognised 

 as a glowing red spot through the external tissues of the abdomen of its host. The differen- 

 tiation of the body occurs in Peltogaster long before the root system has assumed its adult 

 character, so that in this way I have been able to place the identity and nature of the young 

 Peltogaster, before any differentiation of the visceral mass had occurred, with those in which it 

 had already begun, beyond question. 



The differentiation of the organs in Peltogaster internus occurs in the main similarly 

 to that in Sacculina, but there is one exceedingly interesting difference which is illustrated in 

 Piate 6 figs. 4 and 5. Here the fact is shown that no perisomatic cavity, a structure so 

 characteristic of Sacculina interna, is formed. At a corresponding stage in Sacculina the 

 perisomatic cavity would already be complete, because as figs. 4 and 5 show, ali the organs 



