INVERTEBRATE ZOOLOGY IN THREE BOOKS - BOOK 1 - H.Y. Shcherbak - 1995

SUBKINGDOM MULTICELLULAR ANIMALS (METAZOA)

SECTION PROMETAZOA (PROMETAZOA)

PHYLUM RECEPTACULITA (RECEPTACULITA)

The systematic position of this extinct group of animals within invertebrates remained undetermined for a long time. It was previously thought to be closely related to Sponges or Coelenterates. According to modern concepts, Receptaculites constitute a distinct phylum with a unique structural plan, positioned at the organizational level of sponges and archaeocyathids. These fossil benthic marine organisms are known from the Ordovician to the Permian period.

Receptaculites are cup-shaped, radially symmetrical organisms with a calcareous Skeleton, ranging in size from a few millimeters to 30—40 cm. The skeleton consists of outer and inner perforated walls, enclosing a central cavity that forms an opening at the upper part of the animal, possibly an osculum (Fig. 85). Externally, the skeleton is covered by tightly fitting rhombic or hexagonal calcareous plates arranged in regular spiral rows. The outer wall lies beneath this covering. Like the inner wall, it is formed by individual skeletal elements known as meroms. Each merom consists of four rays radiating at right angles to one another in a single plane within the outer wall. These are connected to the corresponding rays of the inner wall by means of a hollow double-walled tube (radiale). The ends of the rays connect with the ends of adjacent meroms to form reticulated outer and inner walls. It is known that receptaculites reproduced by budding; young individuals immediately developed meroms, while the outer covering formed later. Such a method of skeleton formation is unknown in other animals. There are several dozen species within the phylum, grouped into three classes. One of the representative species is Receptaculites neptuni.

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Fig. 85. Diagram of a receptaculitid