NO. 3 DEICHMANN : HOLOTHURIOIDEA ; PART I, DENDROCHIROTA 75 



change considcrabh^ during the animal's life, either becoming more com- 

 plex or degenerating; in some cases they disintegrate almost completely, 

 with the exception of the end plate which normally seems to persist in 

 the species where it is well developed from the beginning; when it is 

 rudimentary in the young individual, it quickly becomes resorbcd. 



In the integument two layers of spicules are typically present, an ex- 

 ternal, the function of which is presumably to give roughness to the skin, 

 and an inner layer, which gives stiffness. The spicules in the outer layer 

 seem definitely fixed in their position, usually with the external side 

 rough with spines, often perforating the skin, while the spicules in the 

 inner layer are able to slide over each other when the animal contracts — 

 as the cards in a deck — and these spicules are therefore more smooth and 

 flat. Either layer or both may be reduced or lacking; the inner layer is 

 never present in very young individuals, while the outer layer frequently 

 is lost or reduced with age. Usually the spicules in the 2 layers are ex- 

 tremely different, in other cases they are less sharply difiFerentiated. Com- 

 mon tj^pes in the external layer are : the table, a basal plate with a num- 

 ber of pillars (1-6) ending in a varying number of teeth, or the basket 

 (or cup) — a hollow body usually with a number of spines developed on 

 the edge. Other types are oval buttons and plates, often with spines or a 

 rough reticulated mass on the external side. The inner layer consists of 

 regular or irregular buttons or plates, smooth or knobbed. Sometimes also 

 large and more complex bodies may be found. 



A few forms have the body covered partly or completely by scales. In 

 that case the external layer of spicules is usually quite insignificant, 

 although it may contain rather interesting types of deposits. 



In the tube feet a large end plate is almost invariably present in the 

 species which have well-developed tails on the calcareous ring. Likewise, 

 a well-developed end plate is usually accompanied by supporting tables 

 with a curved disk and a spire of varying development — sometimes the 

 spire is totally absent except in very young individuals, but it is usually 

 possible to ascertain whether a supporting rod is derived from the table 

 type or not. In more papilliform appendages the end plate is usually 

 small or lacking. 



No end plate, or a vestigial end plate is usually combined with a 

 simple calcareous ring, and the supporting rods are either simple or pro- 

 vided with a third arm, more or less platelike, often irregularly star 

 shaped. 



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